output: don't forbid multiple commits per frame

It's still not possible to commit while a page flip is pending in DRM,
but we don't need to enforce that here and allowing it through the
common interface can be useful for other backends.

This decouples commits from frame scheduling, which is going to make the
new frame schedulers easier to implement.

types/output/output.c

@@ -652,14 +652,6 @@ static uint32_t output_compare_state(struct wlr_output *output,
 static bool output_basic_test(struct wlr_output *output,
 		const struct wlr_output_state *state) {
 	if (state->committed & WLR_OUTPUT_STATE_BUFFER) {
-		// Modesets will block for the previous frame to complete. Regular
-		// page-flips are non-blocking and require the compositor to wait.
-		if (output->frame_pending &&
-				!(state->committed & (WLR_OUTPUT_STATE_ENABLED | WLR_OUTPUT_STATE_MODE))) {
-			wlr_log(WLR_DEBUG, "Tried to commit a buffer while a frame is pending");
-			return false;
-		}
-
 		// If the size doesn't match, reject buffer (scaling is not
 		// supported)
 		int pending_width, pending_height;