vulkan: Add a pipeline cache

vulkan: Add a pipeline cache

Make the display handle the cache, because we only need one.

We store the cache in
  $CACHE_DIR/gtk-4.0/vulkan-pipeline-cache/$UUID.$VERSION
so we regenerate caches for each different device (different UUID) and
each different driver version.

We also keep track of the etag of the cache file, so if 2 different
applications update the cache, we can detect that.
Vulkan allows merging caches, so the 2nd app reloads the new cache file
and merges it into its cache before saving.