Gimp-dds plugin adds alpha channel to originally 24 bit texture when saving to DXT1

```
What steps will reproduce the problem?
1. open a 24 bit texture (for example a bmp)
2. export it to DDS
3. it will be 32 bit

24 bit texture should remain 24 bit.
```

Original issue reported on code.google.com by `txsanyix` on 16 Feb 2015 at 2:31
