compositor: Move buffer damage fields from weston_output to gl-renderer

Move fields current_buffer and buffer_damage out of weston_output into
gl_output_state, since they are actually specific to the renderer.

Also bring back the previous_damage field so that the screenshooter
can get the damage for the previous frame in a renderer independent
way.
diff --git a/src/compositor.h b/src/compositor.h
index e5c579b..e770664 100644
--- a/src/compositor.h
+++ b/src/compositor.h
@@ -160,8 +160,7 @@
 	int32_t mm_width, mm_height;
 	struct weston_border border;
 	pixman_region32_t region;
-	int current_buffer;
-	pixman_region32_t buffer_damage[2];
+	pixman_region32_t previous_damage;
 	int repaint_needed;
 	int repaint_scheduled;
 	struct weston_output_zoom zoom;