gtk-demo: Add shadertoy demo

gtk-demo: Add shadertoy demo

This adds a small demo of using OpenGL shaders, it renders a quad
over the entire widget with a custom fragment shader. The coordinates
and the uniform names are compatible with the ones on shadertoy.com
(although some features, like texture inputs are missing currently).

The default shader in the demo is
https://www.shadertoy.com/view/wsjBD3 which is CC0, so it is
redistributable by Gtk+ (most other shaders are CC-BY-NC-SA which
isn't obviously compatible). I also added a set of buttons loading
a few other CC0 shaders I found.