A few of the squares (most of them, actually) of progs/tests/crossbar are not
rendered correctly.  Annoyingly, the square that is just GL_REPLACE of the 0.5
gray texture is one of the incorrectly rendered squares.  It is slightly too
dark.    This may be an artifact of specifying the textures as RGB888 and the
driver converting them to RGB565.

Regardless, the (1.0 - 0.5) square is too light, and the ((0.5 + 0.5) * 0.5) and
(0.25 + 0.25) * 1.0) squares are much too dark.  The result in every square is
supposed to be RGB = {0.5, 0.5, 0.5}.