This test verifies that the texture system is choosing the right mip
levels.  It uses a special texture, miplevels.tx, that uses color codes
for each mip level, and renders a 256x256 image where the texture
coordinates are exactly aligned to the pixel grid.  In miplevels, the
256x256 level is pure green, the 512x512 level is red, the 128x128 is
blue.  So of course we expect the output of this test to be pure green
(except for the text "256" in the center).