69 * Compares a surface and with reference image data for equality
70 *
71 * \param surface Surface used in comparison
72 * \param referenceSurface Test Surface used in comparison
73 * \param allowable_error Allowable difference (=sum of squared difference for each RGB component) in blending accuracy.
74 *
75 * \returns 0 if comparison succeeded, >0 (=number of pixels for which the comparison failed) if comparison failed, -1 if any of the surfaces were NULL, -2 if the surface sizes differ.