ZBitmap

Copperbox revision 498.

This revision has a new approach to handling index overlays into the array of bitmap data. Different bitmap resolutions have different size requirements for a pixel - for a mono bitmap its a bit, for a 16 bit bitmap it is 3 x 5 bits, etc. I want to keep this data intact rather than marshaling to an unboxed tuples to represent RGB colours, so I need a scheme to allow different indexing according to resolution. Today's code implements a scheme that I hope will be suitable.