Make sure we can use vello in a window for drawing

[jaytaph]

No description provided.

[jaytaph]

It seems a bit hard to do that in an easy way.. Somehow, we must make sure we can use a "surface" and connect that to vello.. however,.. it looks like we should let vello render to a virtual surface, and somehow copy this surface to gtk. It seems a bit inefficient.

There are other systems (like gstreamer and such) that will do drawings directly without cairo's help, so we know it is possible. We haven't found it yet. In the meantime we have a cairo backend renderer which seems to work.