This would previously fail to compile when included in files that do not
include FileUtil.h due to lack of a type declaration.
This moves the constructor and destructor into the cpp file in order to
satisfy the requirements of unique_ptr construction and deletion. That is,
unique_ptr requires a concrete type at the point of construction and
destruction. If the constructor or destructor is left in the header, then
at the point of construction or destruction, IOFile will still be
considered an incomplete type, as unique_ptr's deleter will still only be
able to see the forward declaration, which it can't use.
These functions don't actually depend on any state from the class
instance, so they don't really belong in the header, and are just
an implementation detail.
At first there weren't many enums in Volume.h, but the number has been
growing, and I'm planning to add one more for regions. To not make
Volume.h too large, and to avoid needing to include Volume.h in code
that doesn't use volume objects, I'm moving the enums to a new file.
I'm also turning them into enum classes while I'm at it.
If banner loading fails once, it will very likely fail again.
Setting m_banner_loaded to true when banner loading fails
prevents LoadBannerFile from wasting time if it's called again.
Banner loading requires loading the file system, which takes a
noticeable amount of time, so this matters.