Somehow fix drawTiles when shape has transformations (scaleX, scaleY), non-zero center point or non-zero x or y coordinates.

Without fix:
![drawtiles-bug](https://cloud.githubusercontent.com/assets/85473/5536944/686b8fea-8aaa-11e4-9907-107728cc8d56.gif)
With fix:
![drawtiles-fixed](https://cloud.githubusercontent.com/assets/85473/5536947/6eec9936-8aaa-11e4-9602-57645b8602df.gif)

I use Tilesheet::drawTiles() for my particle engine. Unfortunately, original drawTiles (for html5 target) works correctly only when no scaleX or scaleY applied to parent shape, no center point and shape has x == 0 and y == 0.

This patch is more hack than fix, but it works. Known problem: re-calculated bounds width and height is not correct sometimes, but it doesn't affect visual appearance.