UI: Add support for texture arrays.
* Extend mipmapped texture class to support texture arrays. I've extended this as thoroughly as I could, although there are still some cases where the class may be incomplete (although it shouldn't have regressed in any way) - one instance is reading DDS files. DDS supports arrays, but this class does not support them any better than it did previously.
* Texture Explorer now has a spin box for selecting which item in a texture array to view. The spin box is only enabled if there is more than 1 element in the array.
merge