render: throttle xcursor updates

With a bad behaving client (e.g. 'less' with mouse support enabled),
we can end up with a *lot* of xcursor updates (so much, that we
flooded the wayland socket before we implemented a blocking
wayl_flush()).

Since there's little point in updating the cursor more than once per
frame interval, use frame callbacks to throttle the updates.

This works more or lesslike normal terminal refreshes:

render_xcursor_set() stores the last terminal (window) that had (and
updated) the cursor.

The renderer's FDM hook checks if we have such a pending terminal set,
and if so, tries to refresh the cursor.

This is done by first checking if we're already waiting for a callback
from a previous cursor update, and if so we do nothing; the callback
will update the cursor for the next frame. If we're *not* already
waiting for a callback, we update the cursor immediately.

render.h

@@ -8,10 +8,10 @@ struct renderer;
 struct renderer *render_init(struct fdm *fdm, struct wayland *wayl);
 void render_destroy(struct renderer *renderer);
 
-void grid_render(struct terminal *term);
 void render_resize(struct terminal *term, int width, int height);
 void render_set_title(struct terminal *term, const char *title);
 void render_refresh(struct terminal *term);
+bool render_xcursor_set(struct terminal *term);
 
 void render_search_box(struct terminal *term);