Make screenshots auto-scale and height-for-width

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.

Capture_d_écran_vidéo_de_27-07-2021_20_15_54

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.