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_rd/renderer_scene_render_rd.h

@@ -1456,6 +1456,8 @@ private:
 		float weight;
 	};
 
+	bool low_end = false;
+
 public:
 	/* SHADOW ATLAS API */
 
@@ -1952,6 +1954,8 @@ public:
 
 	void sdfgi_set_debug_probe_select(const Vector3 &p_position, const Vector3 &p_dir);
 
+	bool is_low_end() const;
+
 	RendererSceneRenderRD(RendererStorageRD *p_storage);
 	~RendererSceneRenderRD();
 };