gpu/shaders: update piet-gpu

Changes:
- faster implementation of RGBA output
- fix stroked clips and images

Signed-off-by: Elias Naur <mail@eliasnaur.com>

gpu/shaders/backdrop.comp

@@ -48,13 +48,13 @@ void main() {
     if (element_ix < conf.n_elements) {
         AnnotatedTag tag = Annotated_tag(conf.anno_alloc, ref);
         switch (tag.tag) {
+        case Annotated_Image:
+        case Annotated_BeginClip:
         case Annotated_Color:
             if (fill_mode_from_flags(tag.flags) != MODE_NONZERO) {
                 break;
             }
             // Fall through.
-        case Annotated_Image:
-        case Annotated_BeginClip:
             PathRef path_ref = PathRef(conf.tile_alloc.offset + element_ix * Path_size);
             Path path = Path_read(conf.tile_alloc, path_ref);
             sh_row_width[th_ix] = path.bbox.z - path.bbox.x;