refactor(grid): change schar_T representation to be more compact

Previously, a screen cell would occupy 28+4=32 bytes per cell
as we always made space for up to MAX_MCO+1 codepoints in a cell.

As an example, even a pretty modest 50*80 screen would consume

50*80*2*32 = 256000, i e a quarter megabyte

With the factor of two due to the TUI side buffer, and even more when
using msg_grid and/or ext_multigrid.

This instead stores a 4-byte union of either:
- a valid UTF-8 sequence up to 4 bytes
- an escape char which is invalid UTF-8 (0xFF) plus a 24-bit index to a
  glyph cache

This avoids allocating space for huge composed glyphs _upfront_, while
still keeping rendering such glyphs reasonably fast (1 hash table lookup
+ one plain index lookup). If the same large glyphs are using repeatedly
on the screen, this is still a net reduction of memory/cache
consumption. The only case which really gets worse is if you blast
the screen full with crazy emojis and zalgo text and even this case
only leads to 4 extra bytes per char.

When only <= 4-byte glyphs are used, plus the 4-byte attribute code,
i e 8 bytes in total there is a factor of four reduction of memory use.
Memory which will be quite hot in cache as the screen buffer is scanned
over in win_line() buffer text drawing

A slight complication is that the representation depends on host byte
order. I've tested this manually by compling and running this
in qemu-s390x and it works fine. We might add a qemu based solution
to CI at some point.

src/nvim/ui_compositor.c

@@ -55,7 +55,7 @@ static int msg_current_row = INT_MAX;
 static bool msg_was_scrolled = false;
 
 static int msg_sep_row = -1;
-static schar_T msg_sep_char = { ' ', NUL };
+static schar_T msg_sep_char = schar_from_ascii(' ');
 
 static int dbghl_normal, dbghl_clear, dbghl_composed, dbghl_recompose;
 
@@ -354,7 +354,7 @@ static void compose_line(Integer row, Integer startcol, Integer endcol, LineFlag
       grid = &msg_grid;
       sattr_T msg_sep_attr = (sattr_T)HL_ATTR(HLF_MSGSEP);
       for (int i = col; i < until; i++) {
-        memcpy(linebuf[i - startcol], msg_sep_char, sizeof(*linebuf));
+        linebuf[i - startcol] = msg_sep_char;
         attrbuf[i - startcol] = msg_sep_attr;
       }
     } else {
@@ -363,9 +363,8 @@ static void compose_line(Integer row, Integer startcol, Integer endcol, LineFlag
       memcpy(linebuf + (col - startcol), grid->chars + off, n * sizeof(*linebuf));
       memcpy(attrbuf + (col - startcol), grid->attrs + off, n * sizeof(*attrbuf));
       if (grid->comp_col + grid->cols > until
-          && grid->chars[off + n][0] == NUL) {
-        linebuf[until - 1 - startcol][0] = ' ';
-        linebuf[until - 1 - startcol][1] = '\0';
+          && grid->chars[off + n] == NUL) {
+        linebuf[until - 1 - startcol] = schar_from_ascii(' ');
         if (col == startcol && n == 1) {
           skipstart = 0;
         }
@@ -378,10 +377,10 @@ static void compose_line(Integer row, Integer startcol, Integer endcol, LineFlag
       for (int i = col - (int)startcol; i < until - startcol; i += width) {
         width = 1;
         // negative space
-        bool thru = strequal((char *)linebuf[i], " ") && bg_line[i][0] != NUL;
-        if (i + 1 < endcol - startcol && bg_line[i + 1][0] == NUL) {
+        bool thru = linebuf[i] == schar_from_ascii(' ') && bg_line[i] != NUL;
+        if (i + 1 < endcol - startcol && bg_line[i + 1] == NUL) {
           width = 2;
-          thru &= strequal((char *)linebuf[i + 1], " ");
+          thru &= linebuf[i + 1] == schar_from_ascii(' ');
         }
         attrbuf[i] = (sattr_T)hl_blend_attrs(bg_attrs[i], attrbuf[i], &thru);
         if (width == 2) {
@@ -396,19 +395,18 @@ static void compose_line(Integer row, Integer startcol, Integer endcol, LineFlag
 
     // Tricky: if overlap caused a doublewidth char to get cut-off, must
     // replace the visible half with a space.
-    if (linebuf[col - startcol][0] == NUL) {
-      linebuf[col - startcol][0] = ' ';
-      linebuf[col - startcol][1] = NUL;
+    if (linebuf[col - startcol] == NUL) {
+      linebuf[col - startcol] = schar_from_ascii(' ');
       if (col == endcol - 1) {
         skipend = 0;
       }
-    } else if (col == startcol && n > 1 && linebuf[1][0] == NUL) {
+    } else if (col == startcol && n > 1 && linebuf[1] == NUL) {
       skipstart = 0;
     }
 
     col = until;
   }
-  if (linebuf[endcol - startcol - 1][0] == NUL) {
+  if (linebuf[endcol - startcol - 1] == NUL) {
     skipend = 0;
   }
 
@@ -568,7 +566,7 @@ void ui_comp_msg_set_pos(Integer grid, Integer row, Boolean scrolled, String sep
   if (scrolled && row > 0) {
     msg_sep_row = (int)row - 1;
     if (sep_char.data) {
-      xstrlcpy(msg_sep_char, sep_char.data, sizeof(msg_sep_char));
+      msg_sep_char = schar_from_buf(sep_char.data, sep_char.size);
     }
   } else {
     msg_sep_row = -1;