Implement decals

Also implemented decal atlas, so projectors and other stuff can be added.
Sidenote: Had to make RID hashable, so some unrelated includes changed
in order to include it in hashfuncs.h

servers/rendering/rasterizer_rd/light_cluster_builder.cpp

@@ -39,6 +39,7 @@ void LightClusterBuilder::begin(const Transform &p_view_transform, const CameraM
 	//reset counts
 	light_count = 0;
 	refprobe_count = 0;
+	decal_count = 0;
 	item_count = 0;
 	sort_id_count = 0;
 }