This adds and use
GsPicture which is like
GtkImage but auto scaling and height-for-width.
This requires a recent bugfix from libhandy that hasn't been released yet.
While the screencast is slow, it isn't slow when testing locally. :) And in practice it shouldn't be slower than what was there before: the main difference is we may need to redraw the screenshots when resizing the window, but that's acceptable.