Use alpha images also on linux

[ChristianFeldmann]

Issue: https://github.com/IENT/YUView/issues/404

New way to get the "best" platform image format:

• If no alpha component is needed, we get the same formats as before.
• If alpha is needed, we get a format with alpha. This may not be the fastest to render on some platforms but in that case we want to have alpha.

Test on:

• [x] Windows
• [x] Linux
• [x] Mac