toktx should allow creation of 3D textures with --depth 1

[toomuchvoltage]

Hi there,

Opening this as per my correspondence with @MarkCallow via email. This is effectively similar to #596 but relates to 3D textures created with toktx. You should be able to create 3D textures with --depth 1 similar to how 2D texture arrays can be created with --layers 1. This helps applications that may query texture depth at runtime via textureSize(gsampler3D, ...) and branch behavior based on the .z value in the shader.

Kind regards,
Baktash.