Add texture UVs to geometry builders

Adds texture UVs for polygon and polyline builders, and adds them to the default polygon rendering style. 

For now, texture UVs are fit to complex polygons by finding the axis-aligned bounding box (AABB) of the poly and then "decaling" the texture on top (so not all of the texture will be visible). Possible future options for UV generation include:

* Finding the oriented bounding box (OBB) of the poly.
* An option to keep the scale normalized either across a specific polygon, or a whole feature (or other set of polys), so that the X & Y axes don't distort.

Additionally, in the polygon render style, texture UV generation is optional, set with the ```texcoords``` flag. If texture UVs aren't requested, then they are not added to the vertex layout, so they don't waste space. This allows user-defined styles to use or not use UVs efficiently as needed.