output: fallback to XRGB in output_pick_format

This will be necessary for the primary buffer.

types/wlr_output.c

@@ -1178,32 +1178,41 @@ static struct wlr_drm_format *output_pick_format(struct wlr_output *output,
 		return NULL;
 	}
 
-	uint32_t fmt = DRM_FORMAT_ARGB8888;
+	struct wlr_drm_format *format = NULL;
+	const uint32_t candidates[] = { DRM_FORMAT_ARGB8888, DRM_FORMAT_XRGB8888 };
+	for (size_t i = 0; i < sizeof(candidates) / sizeof(candidates[0]); i++) {
+		uint32_t fmt = candidates[i];
+
+		const struct wlr_drm_format *render_format =
+			wlr_drm_format_set_get(render_formats, fmt);
+		if (render_format == NULL) {
+			wlr_log(WLR_DEBUG, "Renderer doesn't support format 0x%"PRIX32, fmt);
+			continue;
+		}
 
-	const struct wlr_drm_format *render_format =
-		wlr_drm_format_set_get(render_formats, fmt);
-	if (render_format == NULL) {
-		wlr_log(WLR_DEBUG, "Renderer doesn't support format 0x%"PRIX32, fmt);
-		return NULL;
-	}
+		if (display_formats != NULL) {
+			const struct wlr_drm_format *display_format =
+				wlr_drm_format_set_get(display_formats, fmt);
+			if (display_format == NULL) {
+				wlr_log(WLR_DEBUG, "Output doesn't support format 0x%"PRIX32, fmt);
+				continue;
+			}
+			format = wlr_drm_format_intersect(display_format, render_format);
+		} else {
+			// The output can display any format
+			format = wlr_drm_format_dup(render_format);
+		}
 
-	const struct wlr_drm_format *display_format;
-	if (display_formats != NULL) {
-		display_format = wlr_drm_format_set_get(display_formats, fmt);
-		if (display_format == NULL) {
-			wlr_log(WLR_DEBUG, "Output doesn't support format 0x%"PRIX32, fmt);
-			return NULL;
+		if (format == NULL) {
+			wlr_log(WLR_DEBUG, "Failed to intersect display and render "
+				"modifiers for format 0x%"PRIX32, fmt);
+		} else {
+			break;
 		}
-	} else {
-		// The output can display any format
-		display_format = render_format;
 	}
-
-	struct wlr_drm_format *format =
-		wlr_drm_format_intersect(display_format, render_format);
 	if (format == NULL) {
-		wlr_log(WLR_DEBUG, "Failed to intersect display and render "
-			"modifiers for format 0x%"PRIX32, fmt);
+		wlr_log(WLR_ERROR, "Failed to choose a format for output '%s'",
+			output->name);
 		return NULL;
 	}