Add the names of the main widgets as keywords to
our demos, but also things like "game". This helps
finding relevant demos in our growing list. You
can now for example type "label", and find the
"error states" and "links" demos showing GtkLabel
features.
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.
