Framebuffers are getting freed in the GPUContext base class destructor. But
the framebuffer destructors use the MTL/VK/GLContext derived class, whose
destructor has already completed at this point. So these contexts are no
longer valid to use.
Now free the framebuffers earlier.
This caused ASAN warnings, it's not known to cause actual bugs.
Pull Request: https://projects.blender.org/blender/blender/pulls/132504
24e5226ff0