qtdemux: add support for 'vc-1' mapping as found in some ismv files

e.g. To_The_Limit_720_2962.ismv
qtdemux: add support for 'wma ' mapping as found in some ismv files

e.g. To_The_Limit_720_2962.ismv
qtdemux: improve debug logging of fourccs

If we can't show ASCII, at least show them
in big endian order.
shapewipe: Unref caps and element after usage

https://bugzilla.gnome.org/show_bug.cgi?id=734478
speexenc: Improve annotation of internal function

https://bugzilla.gnome.org/show_bug.cgi?id=734542
tests: rtp-payloading: adjust test data to avoid NAL chopping

... and correspondingly unexpected buffer sizes.
configure: use pkg-config to detect x11 and simplify checks

AC_PATH_XTRA macro unnecessarily pulls in libSM and libICE.

https://bugzilla.gnome.org/show_bug.cgi?id=731047
qtdemux: forward DISCONT from upstream to the output streams

This makes sense in DASH reverse playback, where the upstream dashdemux
will download DASH segments in reverse order, but push their buffers
forward to qtdemux and mark each segment start as DISCONT. This needs
to be forwarded downstream to the parser/decoder, otherwise it won't work.

https://bugzilla.gnome.org/show_bug.cgi?id=734443
configure: fix x11 checks to be non-fatal again

Must pass an action-if-not-found argument to
PKG_CHECK_MODULES or it will error out when
it can't find the module requested. Also fix
AC_CHECK_LIB usage, extra libs argument was
in the wrong place.
rtpjpegpay/-depay: Remove incorrectly introduced framesize SDP attribute

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=726415