Updating a texture while it is attached to the currently bound FBO is
something GL implementations tend to get wrong. NVIDIA fails at
glTexSubImage2D(), fglrx and Mesa with glTexImage2D(). I'm afraid to try what
happens on OS X. Fortunately we never use glTexImage2D() while a texture is
attached to an FBO, so we only need to care about glTexSubImage2D().