ignores the depth buffer in this case, but I'm not sure that behaviour
is correct. The spec suggests it should be an INVALID_OPERATION error
(section 4.3.3): "Calling BlitFramebuffer will result in an
INVALID_OPERATION error if mask includes DEPTH_BUFFER_BIT or
STENCIL_BUFFER_BIT, and the source and destination depth and stencil
buffer formats do not match."
I don't mind either way, as long as the behaviour is consistent across both browsers.