this PR is only part of the picture and will indeed help with MMAL decoding … the long term solution the Pi foundation wants to implement is access via /dev/videoX and m2m v4l support:
https://www.raspberrypi.org/forums/viewtopic.php?t=268356#p1628856
james is correct that the camera interface combined with opengl should be enough for the latter (i’m not sure about the state of the implementation in our kernel for v4l m2m though) …
PS: a snap using the MMAL implementation (still waiting for the PR to be merged before it can drop devmode) is at: