Implement shadow meshes

-When importing, a vertex-only version of the mesh is created.
-This version is used when rendering shadows, and improves performance by reducing bandwidth
-It's automatic, but can optionally be used by users, in case they want to make special versions of geometry for shadow casting.

servers/rendering_server.h

@@ -354,6 +354,8 @@ public:
 	virtual void mesh_set_custom_aabb(RID p_mesh, const AABB &p_aabb) = 0;
 	virtual AABB mesh_get_custom_aabb(RID p_mesh) const = 0;
 
+	virtual void mesh_set_shadow_mesh(RID p_mesh, RID p_shadow_mesh) = 0;
+
 	virtual void mesh_clear(RID p_mesh) = 0;
 
 	/* MULTIMESH API */