This is part of the Vulkan task T68990.
This commits changes a few things:
- Rename extensions to capabilities (but left the file name untouched).
- Cubemap mip render workaround detection is rewritten using gl
commands to avoid using the GPU API before initialization.
- Put all the capabilities that are only relevant for the GL backend
inside GLContext as static variables.
- Cleanup the names of the limit variables.
- Separate all GL related workaround search inside the GL module.
Part of the vulkan implementation T68990.
Pretty straight forward. Just move the GL code inside the GLBackend and
make the GPUPlatformGlobal a class object.
