Adding MFCreateSinkWriterFromMediaSink and MFCreateSinkWriterFromURL back to W8CORE.

MFCreateSinkWriterFromMediaSink is supported from Vista all the way to Windows store apps, and Windows Phone 8.1:
http://msdn.microsoft.com/en-us/library/windows/desktop/dd388103(v=vs.85).aspx

I have been working with MFCreateSinkWriterFromMediaSink on an app that I've been testing with Windows 8.1 on an Intel chip, and on a Surface RT.

MFCreateSinkWriterFromURL is supported from Vista all the way to Windows store apps, and Windows Phone 8.1:
http://msdn.microsoft.com/en-us/library/windows/desktop/dd388105(v=vs.85).aspx