DrawableSpans Sanity++

This adds some minor features and fixes some bugs in order to make life easier for folks who are writing exporters with this library. Namely, me.
My thought process here was that the GeoSpans already have bounds, so we shouldn't calculate those unless the user explicitly requests that functionality (ie is cognizant of what's going on). Otherwise, we might just be wasting our time recompulting bounds that are already correct.