auxiliary: move Ben Skeggs' primitive splitter to common code

This is a simple framework that handles splitting primitives in an
abstract way.

The user has to specify the primitive start, start index and count.

Then, it can ask the primitive splitter to "draw" a chunk of the
primitive, staying under a given vertex/index budget.

The primitive splitter will then call user-supplied functions to
emit a range of vertices/indices, as well as switch the edgeflag
on or off.

This is particularly useful for hardware that either has limits
on the vertex count field, or where vertices are pushed on a FIFO
or temporary buffer of limited size.

Note that unlike other splitters, it does not manipulate data in
any way, and merely asks a callback to do so, in vertex intervals.