add API to directly request access to pixbuf
vtl: make videotimeline work with new cairo canvas

timeline thumbnail display, moving (drag/drop) and zoom works.

There still some crashes e.g. resizing the height of the timeline
and with off-screen image buffering when zooming in.
Likely due to concurrency issues:
VideoImageFrame::exposeimg() and direct access of the pixbuf:
"Assertion `!_bounding_box_dirty' failed." in canvas/item.cc:191

more work is needed..