Surface extend modes

[vancluever]

Define extend modes for various surfaces (namely gradients).

This defines what to do if a pixel that's needed goes beyond the bounds of the surface. Examples:

• None: Surface returns blank (black/transparent) pixels outside of the bounds.
• Repeat: Surface repeats/wraps.
• Reflect: Reverse repeat/wrap.
• Pad: Last pixel on the bounds is repeated infinitely. This is pretty much what our gradients do right now.