If we have not yet received our proper draw for the new size
allocation (likely right after the size allocate), then we can
just use the old surface but at a scaled value. This is handy
so that we don't block the main loop trying to do drawing of
lots of data points. Instead we just scale the image and wait
for the high-quality version to complete.
This starts getting the mechanics in place for off screen
rendering using a cairo image surface. We create our own
point cache for storing x,y pairs and then simplify our
drawing based on that.
This provides the plumbing to do the threaded drawing, we just
need to write the capture cursor and draw operations from the
pixman/cairo worker thread (and do so safely).
