Thumbnails with an alpha channel:
Do we want real transparency for thumbnails? I vote "no".

- remove the code that I added to speed up drawing by setting the composition mode to source.
This broke the rating pixmap, and thumbnails with an alpha channel
- In thumbnail creator: images without an alpha channel are not converted to have one.
Images with an alpha channel are composed over white to remove the alpha channel.

TODO: Find out if this is necessary. Find out about FreeDesktop spec.