extend register machine into other contexts

[tonycoz]

Right now the register machine is only used in the transform2() function, but this could be expanded into a more generic mechanism.

The case that made me consider this was flood_fills, the user could pass in a register machine expression for pixels that match flood fillable pixels, or for border pixels (or maybe both).

This will however add the concept of a non-color return value to the register machine, and to anything that needs to implement it (including the considered LLVM implementation.)