Dev vg x11

- add an X11 vg back-end
- imported from github.com/Arrow/vgximg

Instead of copying all this code from vgimg.go, can this share code?
What I had in mind is:
vgx11.Canvas has a vgimg.Canvas field.
When creating a new vgx11.Canvas, pass the ximg to vgimg.NewImage().
Then you can delete most of this code, leaving only the type definition, the New methods, and Paint.
Would that work?