cssimage: Implement proper cross-fades for gradients

cssimage: Implement proper cross-fades for gradients

This is intended mainly to speed up the current situation with spinners
on debug kernels. Because we now don't use a cross-fade to draw the
transition but instead have a real gradient that we draw, we don't need
to use the slow cross-fade code.

https://bugzilla.gnome.org/show_bug.cgi?id=684639