va: Add jpegdecoder base class.

va: Add jpegdecoder base class.

This base class is intented for hardware accelerated decoders, but since
only VA uses it, it will be kept internally in va plugin.

It follows the same logic as the others video decoders in the library but.
as JPEG are independet images, there's no need to handle a DBP so no need
of a picture object. Instead a scan object is added with all the structures
required to decode the image (huffman and quant tables, mcus, etc.).

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1575>