vp8enc: finish support for temporally scaled encoding

vp8enc: finish support for temporally scaled encoding

  - introduce two new properties:

    * temporal-scalability-layer-flags:

      Provide fine-grained control of layer encoding to the
      outside world. The flags sequence should be a multiple of
      the periodicity and is indexed by a running count of encoded
      frames modulo the sequence length.

    * temporal-scalability-layer-sync-flags:

      Specify the pattern of inter-layer synchronisation (i.e.
      which of the frames generated by the layer encoding
      specification represent an inter-layer synchronisation).
      There must be one entry per entry in
      temporal-scalability-layer-flags.

  - apply temporal scalability settings and expose as buffer
    metadata.

    This allows the codec to allocate a given frame to the correct
    internal bitrate allocator. Additionally, all the
    non-bitstream metadata needed to payload a temporally scaled
    stream is now attached to each output buffer as a
    GstVideoVP8Meta.

  - add unit test for temporally scaled encoding.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-good/-/merge_requests/728>