theme: Remove our own gradient stuff

Part one of porting to cairo.

Thanks to Benjamin Otte for helping me clean this up.

https://bugzilla.gnome.org/show_bug.cgi?id=662962
theme: Use cairo for drawing ICON

Thanks to Benjamin Otte for helping me clean this up.

https://bugzilla.gnome.org/show_bug.cgi?id=662962
theme: Use cairo for TINT operations with alpha

https://bugzilla.gnome.org/show_bug.cgi?id=662962
theme: Use cairo for drawing uncolorized IMAGEs

https://bugzilla.gnome.org/show_bug.cgi?id=662962
theme: Remove COLORIZE feature of images

From a quick code search and grep of gnome-themes-standard, none of
the themes that I inspected used this feature. Since it's the last
thing that uses a lot of old legacy GdkPixbuf code, I'd rather just
consider the feature unsupported at this point and clean up everything
I need to.

https://bugzilla.gnome.org/show_bug.cgi?id=662962