mfreadwrite/reader: Create and append a converter transform.
This enables advanced color conversion in all cases, and thus allows
NV12 -> RGB32 conversion even when MF_SOURCE_READER_ENABLE_ADVANCED_VIDEO_PROCESSING
is not set. This should be harmless and makes the code simpler as we
can simply append a VideoProcessor transform in all cases.
The tests todos is tweaked to reflect cases where a single processor
is used, which outputs slightly different attributes to when it is
connected to an upstream decoder. Ultimately we could try to match
native here, but it shouldn't matter too much in the meantime.