backend/drm: strip alpha channel if necessary

Some primary planes don't support ARGB8888, they only support XRGB8888
(for instance on older Intel hardware). The DRM backend would fail to
initialize.

When that's the case, try to allocate buffers without an alpha channel.

backend/drm/renderer.c

@@ -196,9 +196,27 @@ void drm_plane_finish_surface(struct wlr_drm_plane *plane) {
 	finish_drm_surface(&plane->mgpu_surf);
 }
 
+static uint32_t strip_alpha_channel(uint32_t format) {
+	switch (format) {
+	case DRM_FORMAT_ARGB8888:
+		return DRM_FORMAT_XRGB8888;
+	default:
+		return DRM_FORMAT_INVALID;
+	}
+}
+
 bool drm_plane_init_surface(struct wlr_drm_plane *plane,
 		struct wlr_drm_backend *drm, int32_t width, uint32_t height,
 		uint32_t format, uint32_t flags, bool with_modifiers) {
+	if (!wlr_drm_format_set_has(&plane->formats, format, DRM_FORMAT_MOD_INVALID)) {
+		format = strip_alpha_channel(format);
+	}
+	if (!wlr_drm_format_set_has(&plane->formats, format, DRM_FORMAT_MOD_INVALID)) {
+		wlr_log(WLR_ERROR, "Plane %"PRIu32" doesn't support format 0x%"PRIX32,
+			plane->id, format);
+		return false;
+	}
+
 	struct wlr_drm_format_set *format_set =
 		with_modifiers ? &plane->formats : NULL;
 
@@ -270,15 +288,6 @@ bool drm_fb_lock_surface(struct wlr_drm_fb *fb, struct wlr_drm_surface *surf) {
 	return true;
 }
 
-static uint32_t strip_alpha_channel(uint32_t format) {
-	switch (format) {
-	case DRM_FORMAT_ARGB8888:
-		return DRM_FORMAT_XRGB8888;
-	default:
-		return DRM_FORMAT_INVALID;
-	}
-}
-
 bool drm_fb_import_wlr(struct wlr_drm_fb *fb, struct wlr_drm_renderer *renderer,
 		struct wlr_buffer *buf, struct wlr_drm_format_set *set) {
 	drm_fb_clear(fb);