ClutterShaderEffect doesn't require to be sub-classed in order to be
useful. It is possible to just create an instance, set the source and
the uniforms, and attach it to an actor. This should effectively replace
ClutterShader for good.
Sub-classes of ShaderEffect currently have to get the handle for the
Cogl shader and call cogl_shader_source(); this makes it awkward to
implement a ShaderEffect, and it exposes handles and Cogl API that we
might want to change in the future.
We should provide a ClutterShaderEffect method that allows to (safely)
set the shader source at the right time for sub-classes to use.
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.
