Implement handling of video frame transformations upon encoding

QVideoFrame might be mirrored and rotated (QVideoFrame::rotation, QVideoFrame::mirrored).
It's handled properly in frame rendering, see QVideoWindowPrivate::render.

The ffmpeg encoder must consider them and record the frames properly.

Use cases:

• a user sets a custom transformation to video frames and sends them to the media recorder.
• eglfs screen capture generates frames flipped + rotated 180.

Notes:
Write correct side_data to avframes and maybe to stream->codecpar.
Note, that writing the side data to stream->codecpar might be challenging as we don't have the rotation data when we open the codec.

First of all, try modifying frame->side_data with type AV_PKT_DATA_DISPLAYMATRIX.
See our util func streamSideData(...), and ffmpeg funcs av_packet_side_data_add, av_display_rotation_set, av_display_matrix_flip.