Add an element that converts AYUV video frames to a DVB
subpicture stream.

It's fairly simple for now. Later it would be good to support
input via a stream that contains only GstVideoOverlayComposition
meta.

The element searches each input video frame for the largest
sub-region containing non-transparent pixels and encodes that
as a single DVB subpicture region. It can also do palette
reduction of the input frames using code taken from
libimagequant.

There are various FIXME for potential improvements for now, but
it works.

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