unicode-combine: remove utf8proc dependency

We only used utf8proc to try to pre-compose a glyph from a base and
combining character.

We can do this ourselves by using a pre-compiled table of valid
pre-compositions. This table isn't _that_ big, and binary searching it
is fast.

That is, for a very small amount of code, and not too much extra RO
data, we can get rid of the utf8proc dependency.

render.c

@@ -442,7 +442,7 @@ render_cell(struct terminal *term, pixman_image_t *pix,
         }
     }
 
-#if FOOT_UNICODE_COMBINING
+#if FOOT_UNICODE_MAX_COMBINING_CHARS > 0
     /* Combining characters */
     const struct combining_chars *comb_chars = &row->comb_chars[col];
     for (size_t i = 0; i < comb_chars->count; i++) {