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.h

@@ -374,7 +374,7 @@ struct terminal {
         struct {
             struct coord actual;     /* Absolute */
             struct coord in_view;    /* Offset by view */
-            struct cell *cell; /* For easy access to content */
+            struct row *row;         /* Actual row TODO: remove */
         } last_cursor;
 
         struct buffer *last_buf;     /* Buffer we rendered to last time */