First change also avoids a leak of classes (later activates) when the transform is successfully connected.
The use of MFTEnumEx is necessary so that the tests work, as they use a locally registered transform.
-- v2: mfreadwrite/tests: Test the D3D awareness of source reader transforms. mfreadwrite/tests: Test the source reader stream change events. mfreadwrite/reader: Make the GetTransformForStream category parameter optional. mfreadwrite/reader: Use MFTEnumEx to enumerate stream transforms.