gdkpixbufoverlay: add "positioning-mode" property to allow absolute positions

gdkpixbufoverlay: add "positioning-mode" property to allow absolute positions

Set positioning-mode=pixels-absolute to allow positioning with
absolute coordinates, meaning negative x/y offsets will be
interpreted as being to the left/above the video frame instead
of being interpreted as relative to the right/bottom edge of
the video frame (which is a silly default, but that's how it is).

This means we can nicely slide images into and out of the frame,
see gdkpixbufoverlay-test.

https://bugzilla.gnome.org/show_bug.cgi?id=739566