UI: Add support for viewing individual slices of a 3D texture
* This is implemented by overriding the use of 'faces' which is intended for use to support cubemaps. There will be one 'face' for each slice of the 3D texture.
* Also changed the texture explorer so that the order of the spin boxes is sample, array index, slice index. I feel this better represents the likelihood of each being used.
Merge branch 'master' of https://github.com/ValveSoftware/vogl