WMF: fix output samples from our video probe MFTransform.

The EVR sink allocates its buffers and expect the node connected to it
to use them. Our video probe MFTransform should therefore copy the input
buffers into the EVR buffers rather than just passing on the input
buffers.

Task-number: QTBUG-30435
Change-Id: I978479ced341b96ce627c682f99662dec1dfdb3f
Reviewed-by: Christian Stromme <christian.stromme@digia.com>