status, offset, r, g, b, a = cpattern:GetColorStopRGBA(index)
index for a
gradient pattern. Values of index range from 0 to n-1
where n is the number returned
by cpattern:GetColorStopCount().
Note that the color and alpha values are not premultiplied.
This function also returns #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.
#CAIRO_STATUS_SUCCESS or #CAIRO_STATUS_INVALID_INDEX