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.