unicode-combining: store seen combining chains "globally" in the term struct

Instead of storing combining data per cell, realize that most
combinations are re-occurring and that there's lots of available space
left in the unicode range, and store seen base+combining combinations
chains in a per-terminal array.

When we encounter a combining character, we first try to pre-compose,
like before. If that fails, we then search for the current
base+combining combo in the list of previously seen combinations. If
not found there either, we allocate a new combo and add it to the
list. Regardless, the result is an index into this array. We store
this index, offsetted by COMB_CHARS_LO=0x40000000ul in the cell.

When rendering, we need to check if the cell character is a plain
character, or if it's a composed character (identified by checking if
the cell character is >= COMB_CHARS_LO).

Then we render the grapheme pretty much like before.

terminal.c

@@ -822,6 +822,8 @@ term_init(const struct config *conf, struct fdm *fdm, struct wayland *wayl,
         .normal = {.damage = tll_init(), .scroll_damage = tll_init(), .sixel_images = tll_init()},
         .alt = {.damage = tll_init(), .scroll_damage = tll_init(), .sixel_images = tll_init()},
         .grid = &term->normal,
+        .composed_count = 0,
+        .composed = NULL,
         .meta = {
             .esc_prefix = true,
             .eight_bit = true,
@@ -1086,6 +1088,8 @@ term_destroy(struct terminal *term)
     tll_free(term->normal.scroll_damage);
     tll_free(term->alt.scroll_damage);
 
+    free(term->composed);
+
     free(term->window_title);
     tll_free_and_free(term->window_title_stack, free);
 
@@ -2295,10 +2299,6 @@ term_print(struct terminal *term, wchar_t wc, int width)
     cell->wc = term->vt.last_printed = wc;
     cell->attrs = term->vt.attrs;
 
-#if FOOT_UNICODE_MAX_COMBINING_CHARS > 0
-    row->comb_chars[term->grid->cursor.point.col].count = 0;
-#endif
-
     row->dirty = true;
     cell->attrs.clean = 0;