LAYERTYPE_GAME was always unused, as game layers are instead tile layers with the game flag. This fixes the check for consistency, but it wasn't really required anyway, as the UI controls for the group are hidden and it's therefore already impossible to change the group of the game layer.

This makes CTile a struct for consistency, as all other mapitems and the like are also structs. Preparation for #2931.

Under some circumstances the OpenGL package cannot be found when the component specification is used on find_package. For example: Ubuntu 20.04.3 LTS, 64 bit in VirtualBox using afl-clang-fast++ (AFL++ fuzzer) as compiler.