MSMediaFoundation crash if DLL is unloaded at shutdown of itom

[M. Gronle]

It has to be verified if this bug can be reproduced:

If at least one MSMediaFoundation instance has been loaded (and possibly closed again) and itom is closed, the command MFShutdown() in the destructor of class Media_Foundation crashes.

If this can be reproduced, a possible solution would be to not create the singleton instance of Media_Foundation as static variable (where the deletion can not be controlled) but to realize this for example if the MSMediaFoundationInterface instance is deleted.

[M. Gronle]

MSMediaFoundation: plugin refactored: static instances are mostly replaced by shared pointers to avoid for instance a crash when closing itom (fixes issue #7)

→ <<cset 71d5f3236d7e>>