render/pixman: advertise support for linear format modifier

When running with the DRM backend, the Pixman renderer needs to
render the cursor buffer. However, DRM drivers only support linear
buffers for these in general, they don't support implicit modifiers
(aka. INVALID).

Advertise support for LINEAR in the Pixman renderer to fix this.

render/pixman/renderer.c

@@ -523,6 +523,8 @@ struct wlr_renderer *wlr_pixman_renderer_create(void) {
 	for (size_t i = 0; i < len; ++i) {
 		wlr_drm_format_set_add(&renderer->drm_formats, formats[i],
 			DRM_FORMAT_MOD_INVALID);
+		wlr_drm_format_set_add(&renderer->drm_formats, formats[i],
+			DRM_FORMAT_MOD_LINEAR);
 	}
 
 	return &renderer->wlr_renderer;