PVRTC minimum data work count

As detailed here: https://github.com/KhronosGroup/KTX-Specification/issues/131
It appears that a PVRTC surface must always have at least 2x2=4 data words.
Specifically this seems from this: https://github.com/KhronosGroup/KTX-Specification/issues/131#issuecomment-664576977

> From the OpenGL ES `IMG_texture_compression_pvrtc` extension:
>
> > * For PVRTC 4BPP formats the `imageSize` is calculated as:
> > `(max(width, 8) * max(height, 8) * 4 + 7) / 8`
> > * For PVRTC 2BPP formats the `imageSize` is calculated as:
> > `(max(width, 16) * max(height, 8) * 2 + 7) / 8`
>
> Seems like 32 bytes is the minimum PVR image size possible.

This is kind-of ambiguous with this document's phrasing of "At least one word of PVRTC data must exist in each mip level of a texture.", because although it's correct it probably should read "At least four words of PVRTC data must exist in each mip level of a texture because interpolation requires 2x2 data words".