This patch eliminates code duplication between
piglit_probe_image_rgb() and piglit_probe_image_rgba() by implementing
them both in terms of a common function, piglit_probe_image_color().
In addition to being able to test GL_RGB and GL_RGBA,
piglit_probe_image_color() should be able to test any color buffer
format (e.g. GL_RG or GL_LUMINANCE_ALPHA).

piglit_probe_image_color() makes use of a new function,
piglit_num_components(), to determine how many components a given base
internal format contains (e.g. piglit_num_components(GL_RGBA) == 4).
This function is exposed through piglit-util.h in case it can be
useful in writing tests.

Reviewed-by: Anuj Phogat <anuj.phogat@gmail.com>
Acked-by: Marek Olšák <maraeo@gmail.com>