By default, ShaderEffect creates a fragment shader; in order to be able
to deprecate ClutterShader we need a way for ShaderEffect sub-classes to
create a vertex shader if needed - By using a write-only, constructor
only property.

ClutterShader has, internally, a ClutterShaderType enumeration that can
be used exactly for this. We just need to expose it and create a GObject
property for ClutterShaderEffect.