Insufficient validity check for transcodable leads to assert

[MarkCallow]

Discussed in #555

There is a transcodable check at the top of ktxTexture2_TranscodeBasis

    if (colorModel != KHR_DF_MODEL_UASTC
        && This->supercompressionScheme != KTX_SS_BASIS_LZ)
    return KTX_INVALID_OPERATION; // Not in a transcodable format.

which does not check if the colorModel is correct for BASIS_LZ. Further down is an assert checking for MODEL_UASTC when model is not MODEL_ETC1S. A bad input file than therefore cause the debug version of the library to crash.

This will be fixed by validating the color model for BASIS_LZ during during file loading. The assert will also be removed as it doesn't do anything useful.