Update cap_msmf.cpp

Add support for WinRT in the MF capture framework by removing the disallowed calls to enumerate devices and create a sample grabber sink and adding framework for the MediaCapture interface and a custom sink which interfaces with the sample grabber callback interface.  The change requires discussion for making it completely functional as redundancy is required given that if the source is a video file, the old code pathways must be used.  Otherwise all IMFMediaSession, IMFMediaSource, and IMFActivate code must use a MediaCapture code path and all sink code must use the CMediaSink custom sink.

Support for the custom sink is extended not for compatibility as Windows Vista client is a minimum regardless, but because it offers more flexibility, could be faster and is able to be used as an optionally different code path during sink creation based on a future configuration parameter.