Add support for low-end 3D rendering.

-Reduce number of uniform sets from 6 to 4.
-Remove features in low end mode, in order to reduce the number of texture units fit to 16.

servers/rendering/renderer_scene_render.h

@@ -260,6 +260,8 @@ public:
 
 	virtual void sdfgi_set_debug_probe_select(const Vector3 &p_position, const Vector3 &p_dir) = 0;
 
+	virtual bool is_low_end() const = 0;
+
 	virtual void update() = 0;
 	virtual ~RendererSceneRender() {}
 };