iOS: Make sure we're deleting framebuffers in the right context

iOS: Make sure we're deleting framebuffers in the right context

When a QWindow was destroyed, we'd delete its corresponding buffers,
but failed to make the correct EAGLContext current first. This would
result in deleting/invalidating buffers for another window (whatever
window's context was current at the time), and that window would then
seemingly stop rendering anything, and turn black on rotation.