output: add wlr_output_get_primary_formats

This allows compositors to get primary formats without manually
calling wlr_output_impl.get_primary_formats.

For example, the Sway patch for linux-dmabuf feedback [1] needs
this.

[1]: https://github.com/swaywm/sway/pull/6313

include/wlr/types/wlr_output.h

@@ -460,6 +460,16 @@ void wlr_output_lock_software_cursors(struct wlr_output *output, bool lock);
  */
 void wlr_output_render_software_cursors(struct wlr_output *output,
 	pixman_region32_t *damage);
+/**
+ * Get the set of DRM formats suitable for the primary buffer, assuming a
+ * buffer with the specified capabilities.
+ *
+ * NULL is returned if the backend doesn't have any format constraint, ie. all
+ * formats are supported. An empty set is returned if the backend doesn't
+ * support any format.
+ */
+const struct wlr_drm_format_set *wlr_output_get_primary_formats(
+	struct wlr_output *output, uint32_t buffer_caps);
 
 
 struct wlr_output_cursor *wlr_output_cursor_create(struct wlr_output *output);

types/output/output.c

@@ -560,16 +560,8 @@ static bool output_basic_test(struct wlr_output *output) {
 		struct wlr_allocator *allocator = output->allocator;
 		assert(allocator != NULL);
 
-		const struct wlr_drm_format_set *display_formats = NULL;
-		if (output->impl->get_primary_formats) {
-			display_formats =
-				output->impl->get_primary_formats(output, allocator->buffer_caps);
-			if (display_formats == NULL) {
-				wlr_log(WLR_ERROR, "Failed to get primary display formats");
-				return false;
-			}
-		}
-
+		const struct wlr_drm_format_set *display_formats =
+			wlr_output_get_primary_formats(output, allocator->buffer_caps);
 		struct wlr_drm_format *format = output_pick_format(output, display_formats,
 			output->pending.render_format);
 		if (format == NULL) {
@@ -868,3 +860,21 @@ void wlr_output_damage_whole(struct wlr_output *output) {
 
 	pixman_region32_fini(&damage);
 }
+
+const struct wlr_drm_format_set *wlr_output_get_primary_formats(
+		struct wlr_output *output, uint32_t buffer_caps) {
+	if (!output->impl->get_primary_formats) {
+		return NULL;
+	}
+
+	const struct wlr_drm_format_set *formats =
+		output->impl->get_primary_formats(output, buffer_caps);
+	if (formats == NULL) {
+		wlr_log(WLR_ERROR, "Failed to get primary display formats");
+
+		static const struct wlr_drm_format_set empty_format_set = {0};
+		return &empty_format_set;
+	}
+
+	return formats;
+}

types/output/render.c

@@ -51,16 +51,8 @@ static bool output_create_swapchain(struct wlr_output *output,
 	struct wlr_allocator *allocator = output->allocator;
 	assert(allocator != NULL);
 
-	const struct wlr_drm_format_set *display_formats = NULL;
-	if (output->impl->get_primary_formats) {
-		display_formats =
-			output->impl->get_primary_formats(output, allocator->buffer_caps);
-		if (display_formats == NULL) {
-			wlr_log(WLR_ERROR, "Failed to get primary display formats");
-			return false;
-		}
-	}
-
+	const struct wlr_drm_format_set *display_formats =
+		wlr_output_get_primary_formats(output, allocator->buffer_caps);
 	struct wlr_drm_format *format = output_pick_format(output, display_formats,
 		output->render_format);
 	if (format == NULL) {