Revert "gtk3: mate_bg_create*thumbnail should return GdkPixbuf*"

This reverts commit 0eef05307bd0c2acb5c780b8f85a239ca810625d.
Revert "gtk3: GdkRGBA support"

This reverts commit e95d24559060365eb4fee8154ad82434bf3c8a0a.