compositor: finish frame if redraw fails

If we are about to finish a frame, but a redraw is pending and we let the
compositor redraw, we need to check for errors. If the redraw fails and
the backend cannot schedule a page-flip, we need to finish the frame,
anyway.

All backends except DRM use a timer to schedule frames. Hence, they cannot
fail. But for DRM, we need to be able to handle drmModePageFlip() failures
in case access got revoked.

This fixes a bug where logind+drm caused keyboard input to be missed as we
didn't reenable it after a failed page-flip during deactivation.
diff --git a/src/compositor-rdp.c b/src/compositor-rdp.c
index 69f1d04..8a302f8 100644
--- a/src/compositor-rdp.c
+++ b/src/compositor-rdp.c
@@ -302,7 +302,7 @@
 	weston_output_finish_frame(output, msec);
 }
 
-static void
+static int
 rdp_output_repaint(struct weston_output *output_base, pixman_region32_t *damage)
 {
 	struct rdp_output *output = container_of(output_base, struct rdp_output, base);
@@ -324,6 +324,7 @@
 				 &ec->primary_plane.damage, damage);
 
 	wl_event_source_timer_update(output->finish_frame_timer, 16);
+	return 0;
 }
 
 static void