Regression in f43e934e

[wmatyjewicz]

f43e934 seems to introduce artifacts in the compressed output and, in general, decrease PSNR.
Compressing the below picture:

with:
tc -f DXT1 -d picture.ktx picture.png
and then decompressing it with:
decomp picture.ktx decomp.png
generates output with rectangular black artifacts:

Reverting this commit makes the artifacts disappear.

[Mokosha]

I remember making this change for a reason but I have no idea what that is anymore. =/