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

modules/gdscript/gdscript_tokenizer.h

@@ -32,6 +32,7 @@
 #define GDSCRIPT_TOKENIZER_H
 
 #include "core/pair.h"
+#include "core/set.h"
 #include "core/string_name.h"
 #include "core/ustring.h"
 #include "core/variant.h"