added TILE_RECT flag to Tilesheet

I don't like that Tilesheet allows you to draw only added tile with specified origin, i want to draw any tile rect with any origin at any time. That's why i'm making this pull request.
You can see usage example and benchmark in updated Bunnymark test, which can be found here: https://github.com/Beeblerox/nme-bunnymark/tree/drawTileRects
Modified drawing mode shows almost the same perfomance (tested on windows), so i think that it could be safely merged.
I've done appropriate pull request to nme repo as well: https://github.com/haxenme/nme/pull/169

Hope that you'll like this addition