cursor: re-enable NULL buffer for cursor

The previous wlr_output_cursor_set_image() allows setting a NULL buffer for the
cursor to hide it. This functionality was used by sway to hide the cursor,
restore the original semantics by allowing NULL buffers again by avoiding the
wlr_buffer allocation in case we have NULL pixels and handing a NULL wlr_buffer
to wlr_output_cursor_set_buffer().

types/output/cursor.c

@@ -376,15 +376,19 @@ static bool output_cursor_attempt_hardware(struct wlr_output_cursor *cursor) {
 bool wlr_output_cursor_set_image(struct wlr_output_cursor *cursor,
 		const uint8_t *pixels, int32_t stride, uint32_t width, uint32_t height,
 		int32_t hotspot_x, int32_t hotspot_y) {
-	struct wlr_readonly_data_buffer *buffer =
-		readonly_data_buffer_create(DRM_FORMAT_ARGB8888, stride, width, height,
-		pixels);
-	if (buffer == NULL) {
-		return false;
+	struct wlr_buffer *buffer = NULL;
+
+	if (pixels) {
+		struct wlr_readonly_data_buffer *ro_buffer = readonly_data_buffer_create(
+			DRM_FORMAT_ARGB8888, stride, width, height, pixels);
+		if (ro_buffer == NULL) {
+			return false;
+		}
+		buffer = &ro_buffer->base;
 	}
-	bool ok = wlr_output_cursor_set_buffer(cursor, &buffer->base,
-		hotspot_x, hotspot_y);
-	wlr_buffer_drop(&buffer->base);
+	bool ok = wlr_output_cursor_set_buffer(cursor, buffer, hotspot_x, hotspot_y);
+
+	wlr_buffer_drop(buffer);
 	return ok;
 }