The compare pixel function was a static function used internally by
the test_utils_check_* functions. It takes a pointer to a pixel in a
buffer read back from Cogl and compares it with an expected value.
This function could also be useful in tests wanting to check the data
returned from a call to cogl_texture_get_data so we should share it
with the rest of the tests.
https://bugzilla.gnome.org/show_bug.cgi?id=668913
Reviewed-by: Robert Bragg <robert@linux.intel.com>