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.
CID 966234.
CID 966348.
Use pragma once.
Clean up structs/enums.
CID 966060.
Update Changelog.txt.