Various fixes for ffmpeg and v4l2-proxy backends

[aesteve-rh]

A mix of different non-related fixes.

Fixes include:

• New clang include for ffmpeg buildgen
• Bump bindgen version (see https://github.com/Gnurou/v4l2r/pull/40)
• Make ffmpeg send+sync (tried to do my best to write safety comments)
  • This is required because my implementation on rust-vmm requires it
• Small check for create_bufs in v4l2_proxy backend

If count == 0, then ioctl VIDIOC_CREATE_BUFS will set index to the current number of created buffers, and it will check the validity of memory and format.type.

So we do not want to clear buffers when we receive this.

If this PR feels a bit messy, as it includes different unrelated fixes, I will split them.

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[Gnurou]

Thanks, I think this looks mostly good! I hope my safety assumptions for the Sync implementations are correct, but I cannot say I am totally confident about it. The last patch is all the more welcome that I have never ever used createbufs, so it was likely buggy indeed. ><