This fixes all cases where the color profile was broken, and
libpng warns about. It also makes almost all textures indexed
instead of RGB where possible (textures that don't have
semi-transparent pixels).
A deeper, darker level of leaves is created by tiling the texture
2 by 2, reducing brightness and offsetting this.
For a denser leaf appearence with the 'simple leaves' setting.
Also used for acacia bush leaves.