jpegdec: Factor out gst_jpeg_turbo_parse_ext_fmt_convert()

jpegdec: Factor out gst_jpeg_turbo_parse_ext_fmt_convert()

Pull out peer caps checking code into gst_jpeg_turbo_parse_ext_fmt_convert().
This code is used by libjpeg-turbo extras to determine whether peer is capable
of handling buffers into which libjpeg-turbo can directly decode data. This
kind of check must be performed before jpeg_start_decompress() is called in
gst_jpeg_dec_prepare_decode() as well as in gst_jpeg_dec_negotiate(), hence
the common code.

This commit does modify the code a little to make it easier to call from both
call sites without much duplication, hence the extra `if (*clrspc)` test.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gstreamer/-/merge_requests/1687>