gpu: [compute] pre-transform images before rendering

We're about to change the last stage of the compute pipeline to only
accept images, not sampled textures. This change prepares materials
for pixel-aligned image copying by pre-rendering images to a texture,
applying transforms.

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

gpu/shaders/backdrop.comp

@@ -49,7 +49,7 @@ void main() {
         uint tag = Annotated_tag(conf.anno_alloc, ref);
         switch (tag) {
         case Annotated_Fill:
-        case Annotated_FillTexture:
+        case Annotated_FillImage:
         case Annotated_BeginClip:
             PathRef path_ref = PathRef(conf.tile_alloc.offset + element_ix * Path_size);
             Path path = Path_read(conf.tile_alloc, path_ref);