EXT_framebuffer_object doesn't specify if FBOs are shareable between GL
contexts, but ARB_framebuffer_object explicitly prohibits it.

Instead of relying on wglMakeCurrent() to do what we want.

Apparently it's valid to use a D3D device after its window is destroyed.
OpenGL isn't always so forgiving, so this patch is a first step to avoid
making GL calls on a context without window.