Split the input buffer data into decoder units that represent a JPEG
segment. Handle scan decoder unit specifically so that it can include
both the scan header (SOS) but also any other ECS or RSTi segment.

That way, we parse the input buffer stream only once at the gst-vaapi
level instead of (i) in gst_vaapi_decoder_jpeg_parse() to split the
stream into frames SOI .. EOI and (ii) in decode_buffer() to further
determine segment boundaries and decode them.

In practice, this is a +15 to +25% performance improvement.