mipmaps cause texture offset in resulting vtf

When using convert on any file (tga, png, jpg, etc), if you do NOT enable the --no-mips flag, the 3 leftmost columns of pixels are shifted down by one, and it wraps around.

This issue is present in all rgb/gbr/dxt formats

An image with pixels like this:

```
XXXXXXXX
XXXXXXXX
OOOOOOOO
OOOOOOOO
```

Would turn into this:

```
OOOXXXXX
XXXXXXXX
XXXOOOOO
OOOOOOOO
```