PVRTC decoding: incorrect morton offset for non-square textures

[attilaz]

Morton offset should take texture dimensions into account see reflectedMortonOffset function:

https://www.khronos.org/registry/DataFormat/specs/1.3/dataformat.1.3.html#_format_pvrtc1_4bpp

Current implementation https://github.com/bkaradzic/bimg/blob/master/src/image.cpp#L2973 causes crashes/incorrect textures for non-square textures.

I can send PR with the function in specs, but current implementation seems more optimized.

[bkaradzic]

Can you attach PVRTC texture that would crash?

[attilaz]

I had a crash on windows with texturev.

fractal.zip