The ShaderEffect class is an abstract base type for shader-based
effects. GLSL-based effects should be implemented by sub-classing
ShaderEffect and overriding ActorMeta::set_actor() to set the source
code of the shader, and Effect::pre_paint() to update the uniform
values, if any.

The ShaderEffect has a generic API for sub-classes to set the values
of the uniforms defined by their shaders, and it uses the shader
types we defined for ClutterShader, to avoid re-inventing the wheel
every time.