This patch splits off a new piglit-util function,
piglit_compare_images_color(), from the existing
piglit_probe_image_color() function.  The new function compares two
images in memory, so that the caller can take responsibility for
calling glReadPixels().

In addition, this patch creates a function
piglit_compute_probe_tolerance(), which detemines the correct
tolerance thresholds to use when comparing images.

These functions will be needed in order to test proper behaviour of
MSAA with different buffer formats, since the MSAA format test needs
to be able to read the rendered image using glReadPixels, make
adjustments to it in memory, and then compare the results.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>