Fix for texture animation

[davidlyons]

When a texture animation has multiple rows, like the explosion boomer, it was cycling through the frames in the wrong order. This fix starts with the top row and moves down, instead of starting with the bottom row and moving up. Tile 0 is now the top left, rather than the bottom left.

[jachwe]

I think it should be optionalized since it makes sense in the GL World to go y up rather then down. Although i totally agree that most sprite generators do not work that way, which also makes sense in their own context.

But thank you for putting it out there. I actually had the same problem.