ui: use line-based rather than char-based updates in screen.c

Add ext_newgrid and ext_hlstate extensions. These use predefined
highlights and line-segment based updates, for efficiency and
simplicity.. The ext_hlstate extension in addition allows semantic
identification of builtin and syntax highlights.

Reimplement the old char-based updates in the remote UI layer, for
compatibility. For the moment, this is still the default. The bulitin
TUI uses the new line-based protocol.

cmdline uses curwin cursor position when ext_cmdline is active.

src/nvim/highlight_defs.h

@@ -8,6 +8,8 @@
 typedef int32_t RgbValue;
 
 /// Highlighting attribute bits.
+///
+/// sign bit should not be used here, as it identifies invalid highlight
 typedef enum {
   HL_INVERSE     = 0x01,
   HL_BOLD        = 0x02,
@@ -35,6 +37,17 @@ typedef struct attr_entry {
   .cterm_bg_color = 0, \
 }
 
+// sentinel value that compares unequal to any valid highlight
+#define HLATTRS_INVALID (HlAttrs) { \
+  .rgb_ae_attr = -1, \
+  .cterm_ae_attr = -1, \
+  .rgb_fg_color = -1, \
+  .rgb_bg_color = -1, \
+  .rgb_sp_color = -1, \
+  .cterm_fg_color = 0, \
+  .cterm_bg_color = 0, \
+}
+
 /// Values for index in highlight_attr[].
 /// When making changes, also update hlf_names below!
 typedef enum {