Add ST_ClipByRect

[bradh]

This currently falls back to a GEOS implementation for anything more complex than points. I had originally intended to do the linestring implementation too, but it became ugly.

This is a step in the implementation for the MVT geometry conversion, but wanted to avoid that getting too complicated, and it seems like useful functionality as a standalone function.

I'm not especially happy with the "GeosOperations" bridge code, but unsure of a better way. Would definitely appreciate review comments on that.