3D mipmap compression is broken

[solidpixel]

When mipmapping a 3D image with it is possible that the Z dimension in the input data collapses to one for a downscaled image because that happens to be the smallest dimension in the original data.

The current encoder will automatically convert this into a 2D image with an X x Y block size, ignoring the requested 3D block dimension. This breaks mipmapping, because all images in the chain must use the same block size.