[Impeller] Vulkan playgrounds are broken on macOS.

Proposed [fix is here](https://github.com/chinmaygarde/flutter_engine/commit/a04b407deb714a86407f5b536e8dc07b69727e09). Trying to tidy up another patch and profile but creating an issue in the meantime with a workaround.