uvcv4l2_mjpegdemux: Unmap buffer

https://bugzilla.gnome.org/show_bug.cgi?id=699517
uvch264_mjpgdemux: Don't unmap/unref buffer if it's pushed out as-is
uvch264_mjpgdemux: Remove duplicated variables
celt: remove, replaced by opus.
Fix URIs in REQUIREMENTS
mpegvideoparse: do not update caps when bitrate changes

https://bugzilla.gnome.org/show_bug.cgi?id=705035
dvdspu: artifacts when rendering clipped spu

https://bugzilla.gnome.org/show_bug.cgi?id=705039
mpegdefs: Simplify PCR/PTS/DTS => GST conversion macros

We know we will not overflow 64 bits, therefore just use direct
multiplication/division instead of the scale method (trims usage from
50 instruction calls to 2/3).
mpegtspacketizer: More AFC flags

I should have put those in a previous commit. sorry :(
tsdemux: Flush out pending data on EOS

Ensures downstream elements (like decoders) really get all the data
tsdemux: Add more AC3 detection cases

ATSC ac3 streams are always guaranteed to be AC3 if EAC3 descriptor
is not present
If stream registration id is 'AC-3' then it's also guaranteed to be AC3.
Finally if AC3 descriptor is present it's guaranteed to be AC3.

Only silences a warning, but still.
tsdemux: Remove unused macros
h264parse: Show NALU string type in debug messages

If you know the NALU type by heart I tip my hat to you, for the rest of
us mere mortals this is a bit more helpful
docs/spec: Really remove all mentions of CELT plugin
all: Fix for GST_DISABLE_GST_DEBUG

Where applicable, remove methods that don't do anything different than
the default implementation.
mxfdemux: Implement SEGMENT query
tsdemux: Implement SEGMENT query
mpegdemux: Implement SEGMENT query
gst: Don't swap start/stop for negative rates in the SEGMENT query
midi: Fix handling of the SEGMENT query