TODO:

 * Clipping paths

 * Masks

 * Fixing image transforms

 * Image effects/filters

 * Fill patterns

 * Add actual GError support in the loader, rather then the g_warnings
   that we currently use.

On the back burner:

 * Make error messages translatable (requires adding gettext).

Requests from Christian:
 * gaussian blur
 * gtk theme engine supporting automatic mouseover-and-light-up effect
