Do not write texts outside of keys

This is a bit hacky. The main problem is that there is no way to tell
cairo to limit the width. It will wrap text accordingly to width and
height, it will add ellipsizes if it overlow the box, but if a word
width is larger than the box width, it will write it.

To avoid that, I make sure we don't go too much to the left, and I
redraw the background at the right of the keys. This is not visible
cause we damage track correctly the updated buffer coordinates.

I also moved the damage tracking from do_rectangle and draw_text to
higher draw_key and draw_layout.

drw.h

@@ -38,7 +38,7 @@ void drw_over_rectangle(struct drwsurf *d, Color color, uint32_t x, uint32_t y,
                         uint32_t w, uint32_t h);
 
 void drw_draw_text(struct drwsurf *d, Color color, uint32_t x, uint32_t y,
-                   uint32_t w, uint32_t h, const char *label);
+                   uint32_t w, uint32_t h, uint32_t b, const char *label);
 
 uint32_t setup_buffer(struct drwsurf *drwsurf);