Quality switch does nothing when GPU encoding BC7

When trying to GPU encode BC7, -Quality changes nothing;

```
compressonatorcli.exe   biomech_bk.png   biomech.DDS   -fd BC7  -EncodeWith  GPU  -Quality 1.0000  -NumThreads 8 
```

The result will be the same regardless of what -Quality is set to, but it does work properly with -EncodeWith CPU.