support additive blending for drawTiles() (matches CPP behavior...other blend modes still dont work for native targets so only additive blend was added so far)
pre-multiplied alpha to get rid of black line between textures: http://www.kirupa.com/forum/showthread.php?378202-Alpha-blending-appears-dark-when-using-Stage3D
Merge pull request #4 from XenizoGames/master

support additive blending for drawTiles() (matches native behavior)