Allow user data pass through

[carlosfg-flu]

It would be useful to link some user data to an incoming frame and be able to recover it at the output. This will make integration with frameworks like FFmpeg [1] and GStreamer [2] more seamless.

A way to achieve this would be to add a void pointer to the vvdecAccessUnit structure, which could be recovered from the corresponding vvdecFrame. Something similar to what x265 encoder has here.

[1]: FFmpeg has this opaque feature
[2]: GStreamer has GstMeta, used by GstVideoEncoder base class.