Defines a blend from the current fill color to another color. For axial shadings, (x0, y0) and (x1, y1) are the coordinates of the starting and ending points of the shading. For radial shadings these points specify the centers of the starting and ending circles. c1, c2, c3, and c4 are the color values of the shading endpoint, interpreted in the current fill color space in the same way as the color parameters in pdf_setcolor().
This function returns a shading handle that can be used in subsequent calls to pdf_shading_pattern() and pdf_shfill().
This function requires PDF 1.4 or above.