Follow-up to !8184.
This also adds a bunch of helpers for writing media source tests (without involving the mf session) in mfsrcsnk, these could be helpful for more tests of this kind in the future. I'm somewhat unsure as to why we haven't done tests for source behavior so far: while those are plugins, applications rely on a bunch of things in the builtin sources that microsoft provides.
Since this affects both mfsrcsnk and winegstreamer sources, tests should be run both with `HKCU\Software\Wine\MediaFoundation\DisableGstByteStreamHandler` enabled and disabled. Due to this I figured it'd make sense to implement everything first and then remove the test todo_wine's and the statements rejecting the thin parameter in a single commit ("Allow thinning"). Perhaps this could be solved more nicely by setting the registry key in the tests themselves? I'm unsure if that is something we do in tests, generally.
`MEStreamThinMode` events need to be emitted between the last sample using the outdated thinning parameter and the first sample using the updated thinning parameter. The winegstreamer implementation for this turned out a bit complex, if there is a simpler way to do this please let me know.
Regarding winegstreamer, note that buffers need to be intercepted before the decoder because decoders often discard `GST_BUFFER_FLAG_DELTA_UNIT` flags (which is already annoying in itself - it causes all samples to be marked as `MFSampleExtension_CleanPoint`, this should potentially be worked around in the future). But even besides that, intercepting before the decoder is the "proper" implementation, since the point of thinning is increasing decoding speed by skipping delta frames, tho there are some games that rely on the semantics as well.
-- v5: mfsrcsnk: Emit MEStreamThinMode event. mfsrcsnk: Move media_source_send_sample. mfsrcsnk: Implement thinning. winedmo: Fall back to dts for sample time if pts is not present.