FakeFilterMapper hooks: Document the check for "negative" registry key handles.
FakeFilterMapper: Don't call Init() unless we are sure it's safe to do so.
Improve the Get(Cur)DispMode and SetDispMode functions.

Fixes CID #966222.
Use conversion constructors to work-around confusing AStyle styling.
Use a bitcheck instead of casting (WIP).

Fixes CID #965944.