* Prefetch capabilities before spawning translation threads.
The Backend Multithreading only expects one thread to submit commands at a time. When compiling shaders, the translator may request the host GPU capabilities from the backend. It's possible for a bunch of translators to do this at the same time.
There's a caching mechanism in place so that the capabilities are only fetched once. By triggering this before spawning the thread, the async translation threads no longer try to queue onto the backend queue all at the same time.
The Capabilities do need to be checked from the GPU thread, due to OpenGL needing a context to check them, so it's not possible to call the underlying backend directly.
* Initialize the capabilities when setting the GPU thread + missing call in headless
* Remove private variables
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