tui: Improvements to RGB colour support.

The details are in the on-line help under :help true-color .

The brief precis is that nvim is (I hope.) converging with tmux and libvte.
It is taking the same approach with setrgbf and setrgbb terminfo capabilities
that it does with the Ss and Se terminfo capabilities.

runtime/doc/term.txt

@@ -82,11 +82,16 @@ operation; and it will override |terminfo| saying that it has fewer colours
 available.
 
 						*true-color* *xterm-true-color*
-Nvim supports using true (24-bit) colours in the terminal.  |terminfo| does
+Nvim supports using true (24-bit) colours in the terminal, on terminals that
-not contain flags to say when terminals have true colour support.  So Nvim
+support it.  It uses the same |terminfo| extensions that were proposed by
-simply assumes true colour support for (all) "xterm", "rxvt", "linux",
+Rüdiger Sonderfeld in 2013 for this: "setrgbf" and "setrgbb".  If your
-"putty", and "iterm" terminal types, or when Konsole or a terminal emulator
+terminfo definition specifies these, then nothing more is required.
-that sets the COLORTERM environment variable to "truecolor" is detected.
+
+If your terminfo definition is missing them, then Nvim will on a wide range of
+terminals resort to using the ISO 8613-6:1994/ITU T.416:1993 control sequences
+for setting RGB colours.  This includes the "rxvt", "linux", "st", and "iterm"
+terminal types, or when Konsole, genuine Xterm, or a terminal emulator that
+sets the COLORTERM environment variable to "truecolor" is detected.
 
 							*xterm-resize*
 Nvim can resize the terminal display on some terminals that implement an
@@ -99,12 +104,14 @@ Nvim will adjust the shape of the cursor from a block to a line when in insert
 mode (or as specified by the 'guicursor' option), on terminals that support
 it.  It uses the same |terminfo| extensions that were pioneered by tmux for
 this: "Ss" and "Se".  If your terminfo definition specifies these, as some
-(such as "xterm+tmux") do, then nothing more is required.
+(such as those based upon "xterm+tmux") do, then nothing more is required.
 
 If your terminfo definition is missing them, then Nvim will on a wide range of
 terminals resort to using the conventional DECSUSR control sequence for
 adjusting the cursor shape.  If Konsole is detected, Nvim will use the
-idiosyncratic Konsole terminal control sequences for this.
+idiosyncratic Konsole terminal control sequences for this.  Similarly if the
+Linux kernel's built-in terminal emulator is detected, with its idiosyncratic
+control sequence.
 
 Note: tmux itself accepts the conventional DECSUSR control sequence, the same
 as many other terminals do.  It has to translate this into whatever control

src/nvim/tui/tui.c

@@ -2447,7 +2447,7 @@ static void patch_terminfo_bugs(TUIData *data, const char *term,
 
   // Dickey ncurses terminfo has included the Ss and Se capabilities, pioneered
   // by tmux, since 2011-07-14.  So adding them to terminal types, that do
-  // actually have such control sequences but lack the currect definitions in
+  // actually have such control sequences but lack the correct definitions in
   // terminfo, is a fixup, not an augmentation.
   data->unibi_ext.reset_cursor_style = unibi_find_ext_str(ut, "Se");
   data->unibi_ext.set_cursor_style = unibi_find_ext_str(ut, "Ss");
@@ -2537,6 +2537,7 @@ static void augment_terminfo(TUIData *data, const char *term,
     const char *colorterm, long vte_version, bool konsole, bool iterm)
 {
   unibi_term *ut = data->ut;
+  bool true_xterm = !!os_getenv("XTERM_VERSION");
   bool xterm = term && STARTS_WITH(term, "xterm");
   bool dtterm = term && STARTS_WITH(term, "dtterm");
   bool linuxvt = term && STARTS_WITH(term, "linux");
@@ -2544,6 +2545,7 @@ static void augment_terminfo(TUIData *data, const char *term,
   bool teraterm = term && STARTS_WITH(term, "teraterm");
   bool putty = term && STARTS_WITH(term, "putty");
   bool screen = term && STARTS_WITH(term, "screen");
+  bool st = term && STARTS_WITH(term, "st");
   bool tmux_wrap = screen && !!os_getenv("TMUX");
   bool truecolor = colorterm
     && (0 == strcmp(colorterm, "truecolor") || 0 == strcmp(colorterm, "24bit"));
@@ -2561,13 +2563,43 @@ static void augment_terminfo(TUIData *data, const char *term,
     data->unibi_ext.reset_scroll_region = (int)unibi_add_ext_str(ut, NULL,
       "\x1b[r");
   }
-  // See https://gist.github.com/XVilka/8346728 for more about this.
+
-  if (putty || xterm || rxvt || linuxvt || konsole || iterm || truecolor) {
+  // Dickey ncurses terminfo does not include the setrgbf and setrgbb
-    data->unibi_ext.set_rgb_foreground = (int)unibi_add_ext_str(ut, NULL,
+  // capabilities, proposed by Rüdiger Sonderfeld on 2013-10-15.  So adding
+  // them to terminal types, that do actually have such control sequences but
+  // lack the correct definitions in terminfo, is an augmentation, not a
+  // fixup.  See https://gist.github.com/XVilka/8346728 for more about this.
+  bool has_standard_rgb = vte_version >= 3600  // per GNOME bug #685759
+    || true_xterm;
+  // "standard" means using colons like ISO 8613-6:1994/ITU T.416:1993 says.
+  bool has_non_standard_rgb =
+    linuxvt     // Linux 4.8+ supports true-colour SGR.
+    || konsole  // per commentary in VT102Emulation.cpp
+    // per http://lists.schmorp.de/pipermail/rxvt-unicode/2016q2/002261.html
+    || rxvt
+    || st       // per experimentation
+    || iterm || truecolor;
+  data->unibi_ext.set_rgb_foreground = unibi_find_ext_str(ut, "setrgbf");
+  if (-1 == data->unibi_ext.set_rgb_foreground) {
+    if (has_standard_rgb) {
+      data->unibi_ext.set_rgb_foreground = (int)unibi_add_ext_str(ut, "setrgbf",
+          "\x1b[38:2:%p1%d:%p2%d:%p3%dm");
+    } else if (has_non_standard_rgb) {
+      data->unibi_ext.set_rgb_foreground = (int)unibi_add_ext_str(ut, "setrgbf",
           "\x1b[38;2;%p1%d;%p2%d;%p3%dm");
-    data->unibi_ext.set_rgb_background = (int)unibi_add_ext_str(ut, NULL,
+    }
+  }
+  data->unibi_ext.set_rgb_background = unibi_find_ext_str(ut, "setrgbb");
+  if (-1 == data->unibi_ext.set_rgb_background) {
+    if (has_standard_rgb) {
+      data->unibi_ext.set_rgb_background = (int)unibi_add_ext_str(ut, "setrgbb",
+          "\x1b[48:2:%p1%d:%p2%d:%p3%dm");
+    } else if (has_non_standard_rgb) {
+      data->unibi_ext.set_rgb_background = (int)unibi_add_ext_str(ut, "setrgbb",
           "\x1b[48;2;%p1%d;%p2%d;%p3%dm");
     }
+  }
+
   if (iterm) {
     data->unibi_ext.set_cursor_color = (int)unibi_add_ext_str(
         ut, NULL, TMUX_WRAP(tmux_wrap, "\033]Pl%p1%06x\033\\"));