Developer request: Add a function to insert arbitrary sprites at any position in the current segment.

[realSquidCoder]

Stonesense needs a way to easily add an arbitrary sprite to a position in the segment. We can draw things in the right location, but they're layered wrong. What we need is to lie to the rendering and say "yeah this is a tile that's here." This would properly fix https://github.com/DFHack/stonesense/issues/195