Creates a shading, which can be used by ps_shfill() or ps_shading_pattern().
The color of the shading can be in any color space except for pattern.
The type of shading can be either radial or axial. Each shading starts with the current fill color and ends with the given color values passed in the parameters c1 to c4 (see ps_setcolor() for their meaning).
The coordinates x0, y0, x1, y1 are the start and end point of the shading.
See ps_setcolor() for their meaning.
If the shading is of type radial the optlist must also contain the parameters r0 and r1 with the radius of the start and end circle.