R8G8B8 still contains alpha

```
What steps will reproduce the problem?
1. Given 4K gimp image with alpha, gimp 2.8.14 x64, gimp-dds-win64-3.0.1
2. Save the image in R8G8B8
3. Output file size is ~65MB and should be 49MB

Thank you

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Original issue reported on code.google.com by `slifel...@gmail.com` on 1 Jan 2015 at 10:05
