Blender Git Loki

Fix T47354: Garbage key events (UNKOWNKEY) would remain in backup event's keymodifier.

For some reason, using 'dead keys' to compose some accentuated latin chars will generate
some 'unknown' key events, and in this case, direct `event.keymodifier` was correctly
cleared, but not its 'backup' version in `win->eventstate`, so all further events would
get an invalid modifier until some real one would be pressed again...