Per comments on https://phabricator.services.mozilla.com/D268499 from :alwu and :sotaro we should implement the zero-copy fast path for P010 video decoder output subtype.

Zero-copy avoids a copy from the video decoder output buffer to the video processor input buffer, since both are simply D3D11 textures, other user agents do not bother copying and we should not either (the non-zero-copy code path should be removed eventually if zero-copy proves completely stable).

Currently the zero-copy code makes a bunch of assumptions about NV12 being the only format it will encounter, so it will take a bit of refactoring.

Relevant code starting points:
https://searchfox.org/firefox-main/search?q=symbol:_ZN7mozilla18WMFVideoMFTManager12InitInternalEv&redirect=false
https://searchfox.org/firefox-main/search?q=symbol:_ZN7mozilla18FFmpegVideoDecoderILi46465650EE16CreateImageD3D11ExxxR8nsTArrayI6RefPtrINS_9MediaDataEEE&redirect=false