index for a
gradient pattern. Values of index are 0 to 1 less than the number
returned by cairoPatternGetColorStopCount.cairoPatternGetColorStopRgba(pattern, index)CAIRO_STATUS_SUCCESS, or CAIRO_STATUS_INVALID_INDEX
if index is not valid for the given pattern. If the pattern is
not a gradient pattern, CAIRO_STATUS_PATTERN_TYPE_MISMATCH is
returned.NULLNULLNULLNULLNULL