This test compares rendering to the window system framebuffer with
rendering to an FBO, and it does so by rendering rotated quads. There
can be slight differences in rounding if the Y axis is flipped between
those buffers causing rendering differences which are permitted by GL.

Change the test to render axis-aligned quads in each viewport instead.
This should still catch any error with flipped Ys, and there are no
rounding issues since pixels are fully covered by quads.

Reviewed-by: Brian Paul <brianp@vmware.com>