lib: Deprecate GdkPixdata

GdkPixdata has a number of problems, including:
- it doesn't use the original files, losing metadata
- its RLE compression is ineffecient, even more so for lossy formats
- the way it's implemented doesn't support data > 64 kB on Windows
  (See https://bugzilla.gnome.org/show_bug.cgi?id=603706)
doc: Fix comment characters in GdkPixdata API docs