vrend: Do not cache frame buffer object in vrend resource

vrend resource could cache a frame buffer object to read a texture back. However, vrend resource is independent of OpenGL context while a frame buffer object is not because it is a container object. On QEMU, a frame buffer object is typically cached on each renderer context while its destruction operation is performed on "0 context", resulting in destroying another frame buffer object whose name is identical but is created in "0 context".

This change simply removes the caching mechanism and eliminates the need of a context switch when destroying a cached frame buffer object and some handling of cache misses.

Signed-off-by: Akihiko Odaki akihiko.odaki@gmail.com