While there is a style "background color", the idea of a "widget
background color" is completely bogus, the widget background can be a
gradient or whatever.
- Get rid of "background" in GimpViewable's preview API, only leave the
"foreground color" there for things like brushes or fonts.
- In GimpViewRenderer, add the background types to be used to class and
instance, so each renderer type can choose what it needs.
- Render all previews to alpha surfaces, and do the background
for all renderers generically in gimp_view_renderer_real_draw(),
then render the preview surface on top of it.
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