Currently float and int are supported by default. vec2, vec3, vec4
and mat4 are supported if graphene is used. Of course if one wants
to set custom uniforms they can also be set using the create-shader
signal.
One may not have an GstGLContext available or current in the thread where one
would need to update the shader. Support this by signalling create-shader
whenever the one-shot 'update-shader' is set to TRUE.