render: render combining characters

This is basically just a loop that renders additional glyphs on top
off the base glyph.

Since we now need access to the row struct, for the combining
characters, the function prototype for 'render_cell()' has changed.

When rendering the combining characters we also need to deal with
broken fonts; some fonts use positive offsets (as if the combining
character was a regular character), while others use a negative
offset (to be used as if you had already advanced the pen position).

We handle both - a positive offset is rendered just like a regular
glyph. When we see a negative offset we simply add the width of a cell
first.

terminal.c

@@ -1319,7 +1319,7 @@ term_reset(struct terminal *term, bool hard)
     tll_free(term->normal.scroll_damage);
     tll_free(term->alt.damage);
     tll_free(term->alt.scroll_damage);
-    term->render.last_cursor.cell = NULL;
+    term->render.last_cursor.row = NULL;
     term->render.was_flashing = false;
     term_damage_all(term);
 }