ProxySourceCache

Proxy source cache gets ideal screen tile cover coordinates. All the other source caches's coordinates get mapped to subrects of proxy coordinates (or vice versa, subrects of larger tiles from all source caches get mapped to full proxy tile). This happens on every draw call in Terrain.updateTileBinding. Approach is used here for terrain : all the visible source tiles of all the source caches get rendered to proxy source cache textures and then draped over terrain. It is in future reusable for handling overscalling as buckets could be constructed only for proxy tile content, not for full overscalled vector tile.