virtio-gpu: reset gfx resources in main thread

virtio-gpu: reset gfx resources in main thread

Calling OpenGL from different threads can have bad consequences if not
carefully reviewed. It's not generally supported. In my case, I was
debugging a crash in glDeleteTextures from OPENGL32.DLL, where I asked
qemu for gl=es, and thus ANGLE implementation was expected. libepoxy did
resolution of the global pointer for glGenTexture to the GLES version
from the main thread. But it resolved glDeleteTextures to the GL
version, because it was done from a different thread without correct
context. Oops.

Let's stick to the main thread for GL calls by using a BH.

Note: I didn't use atomics for reset_finished check, assuming the BQL
will provide enough of sync, but I might be wrong.

Acked-by: Dongwon Kim <dongwon.kim@intel.com>
Signed-off-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Message-Id: <20230726173929.690601-3-marcandre.lureau@redhat.com>