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.h

@@ -77,21 +77,16 @@ struct damage {
     int lines;
 };
 
-#if FOOT_UNICODE_MAX_COMBINING_CHARS > 0
-struct combining_chars {
+struct composed {
+    wchar_t base;
+    wchar_t combining[5];
     uint8_t count;
-    wchar_t chars[FOOT_UNICODE_MAX_COMBINING_CHARS];
-} __attribute__((packed));
-#endif
+};
 
 struct row {
     struct cell *cells;
     bool dirty;
     bool linebreak;
-
-#if FOOT_UNICODE_MAX_COMBINING_CHARS > 0
-    struct combining_chars *comb_chars;
-#endif
 };
 
 struct sixel {
@@ -221,6 +216,10 @@ struct terminal {
     struct grid alt;
     struct grid *grid;
 
+    #define COMB_CHARS_LO 0x40000000ul
+    size_t composed_count;
+    struct composed *composed;
+
     struct fcft_font *fonts[4];
     int font_dpi;
     int font_adjustments;