Add GskGLShaderNode and GskGLShader

Add GskGLShaderNode and GskGLShader

A GskGLShader is an abstraction of a GLSL fragment shader that
can produce pixel values given inputs:
 * N (currently max 4) textures
 * Current arguments for the shader uniform
Uniform types are: float,(u)int,bool,vec234)
There is also a builder for the uniform arguments which are
passed around as immutable GBytes in the built form.

A GskGLShaderNode is a render node that renders a GskGLShader inside a
specified rectangular bounds. It renders its child nodes as textures
and passes those as texture arguments to the shader. You also pass it
a uniform arguments object.