Seamless support for multi-row animation

Seamless support for multi-row animation, requiring no different or additional function calls (the addon handles it).

You can try it out with the example that already comes with the addon. Just change the animation's total_frames property from 4 (one character's worth of frames) to 8 (two characters) or 12 (three) or 16 (four), and in the loadTexture function call that loads the sprite sheet, change the texture size from 256 to 128 (since the sprites only take up half the sheet width). Then run it and watch each person transform from character to character.