backend-drm/state-propose: Amend an older comment

Since b38b735e20, 'backend-drm: Remove Pixman conditional
for keep_buffer' the Pixman renderer keeps its own reference to buffers
when attached to surfaces, rather than flipping keep_buffer variable for
the surface. Problem is that when switching from the Pixman render to
the GL would not work and could result in a crash upon first repaint.

Signed-off-by: Marius Vlad <marius.vlad@collabora.com>

libweston/backend-drm/state-propose.c

@@ -981,10 +981,11 @@ drm_assign_planes(struct weston_output *output_base)
 		/* Test whether this buffer can ever go into a plane:
 		 * non-shm, or small enough to be a cursor.
 		 *
-		 * Also, keep a reference when using the pixman renderer.
+		 * FIXME: Also, we should keep a reference when using the
-		 * That makes it possible to do a seamless switch to the GL
+		 * pixman renderer.  That makes it possible to do a seamless
-		 * renderer and since the pixman renderer keeps a reference
+		 * switch to the GL renderer and since the pixman renderer
-		 * to the buffer anyway, there is no side effects.
+		 * keeps a reference to the buffer anyway, there is no side
+		 * effects.
 		 */
 		ev->surface->keep_buffer = false;
 		if (weston_view_has_valid_buffer(ev)) {