render: get rid of 'all-clean' detection

Instead of trying to figure out if we had to render
something (i.e. something in the grid was dirty), and using that to
determine whether to post a callback or not, we now let
render_refresh() set a flag indication we need to render another
frame.

This simplifies render_grid(), which now _always_ renders, and pushes
it to the compositor.

The callback handler checks the pending flag and simply doesn't call
render_grid() when there's no more pending state to render.

This ends up reducing the number of wakeups when e.g. having a
blinking cursor.

terminal.h

@@ -298,6 +298,7 @@ struct terminal {
             struct cell *cell; /* For easy access to content */
         } last_cursor;
 
+        bool pending;
         struct buffer *last_buf;     /* Buffer we rendered to last time */
         bool was_flashing;           /* Flash was active last time we rendered */
         bool was_searching;