Tilesheet getter functions

Added getTileCenter, getTileRect and getTileUVs to reflect openfl.display.Tilesheet.
This necessitates adding haxe arrays for tiles, points and uvs, but adding tiles is rare and overhead is negligible.
Upside is Tilesheets are far less opaque. Downside is users can shoot themselves in the foot if they want, or data can potentially desync between nme and openfl.