Angle, for instance, which is now the default on Windows,
does not support the GL_BGRA_EXT format as input for
glTexSubImage2D(). In the case where it's not detected, we
therefore need to flip the bytes in the input.

Change-Id: Ibe78d0223e3c2c39cb1943cdcf67103044d00aa7
Reviewed-by: Samuel Rødal <samuel.rodal@digia.com>