This adds a bunch of snazz to the gltransitions demo. It is perhaps
a bit overloaded now, but it demos everything that we can do.
Changes:
* The fire shader is now not a bin, it just renders an animating
background with no textures involved.
* The stacks don't all start on the same page.
* The shaderbin passes the mouse coordinate to the shader.
* The shaderbin allows specifying a "border" so that you can
cause effects outside the bin child (something that is new to gtk4).
* All the buttons and the stacks are now in shader-bins that runs
a wobbly-widget effect based on the mouse position that
wobbles outside the child allocation.
Add adds a demo showing off GskGLShaderNode in various ways.
It has a transistion widget, using some examples from
gl-transitions.com, with child widgets being both images, a GL area
and real widgets (that let you edit the transition shaders
themselves.
It also has a fancy fire effect on hove on the buttons.
