FakeFilterMapper: EnumMatchingFilters was always returning E_NOTIMPL.

A "return" statement was missing.
FakeFilterMapper: Various cosmetics.

Make sure that static member variables are easy to spot using the "s_" prefix.
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 a bitcheck instead of casting (WIP).

Fixes CID #965944.