matroska: factor out read context init/reset

While at this, move _track_reset() to track-ids
so it can be called from the common read context
reset routine.

https://bugzilla.gnome.org/show_bug.cgi?id=722705
qtmux: add subtitle support to qtmuxmap structures

adds basic stubs for subtitle support around the qtmux and
qtmuxmap structures. Still no real subtitle implemented, but
basic functions in place

https://bugzilla.gnome.org/show_bug.cgi?id=581295
qtmux: add support for the TX3G atoms

Adds functions for creating and setting values related to the
tx3g atom for raw text subtitle support.

QTFF spec has information on those atoms

https://bugzilla.gnome.org/show_bug.cgi?id=581295
qtmux: add support for text/x-raw subtitles

Adds it to mp4mux, qtmux and gppmux.

Buffers need to be prefixed with 2 bytes for the text length before
being muxed.

https://bugzilla.gnome.org/show_bug.cgi?id=581295
qtmux: improve support for sparse streams

Do not try to use subsequent buffer timestamps to calculate
sparse streams durations because the stream is sparse and
the buffers might not be 'time adjacent'. So rely on the
duration and give the option to the pad to provide
custom 'empty' buffers to represent the gaps in the
stream, this can vary on how the data is represented.

Right now, the only sparse stream supported is tx3g subtitles.
qtmux: remove have_dts flag from pads

It was used in the past in 0.10 when there was no explicit DTS
field in buffers, now we have it in 1.x series and we can
check it directly with GST_BUFFER_DTS_IS_VALID
flx: Fix typo in header include guard

error: '__GST_FLX_FMT__H__' is used as a header guard here,
followed by #define of a different macro [-Werror,-Wheader-guard]
rtpac3depay: Remove unused variable
rtprtxsend: Fix unitialized variable compiler warning

variable 'rtx_ssrc' is used uninitialized whenever
'if' condition is false [-Werror,-Wsometimes-uninitialized]
oss4: Fix typo in header include guard

error: 'GST_OSS4_AUDIO_H' is used as a header guard here,
followed by #define of a different macro [-Werror,-Wheader-guard]
id3v2mux: Fix unitialized variable compiler warning

error: variable 'image_type' is used uninitialized
whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
interleave: Fix unitialized variable compiler warning in test

error: variable 'mask' is used uninitialized
whenever 'if' condition is false [-Werror,-Wsometimes-uninitialized]
souphttpsrc: Fix implicit enum conversion compiler warning

error: implicit conversion from enumeration type
'SoupStatus' to different enumeration type 'SoupKnownStatusCode'
id3v2mux: Fix another compiler warning
matroskaparse: Comment out some unused functions used only from the commented out pull-mode code
goom: Remove unused functions
souphttpsrc: Don't duplicate status_code to GStreamer error mapping
souphttpsrc: Add mapping for NOT_FOUND and NOT_AUTHORIZED errors
matroska: add support for GRAY8, BGR and RGB video colourspaces in V_UNCOMPRESSED codec

https://bugzilla.gnome.org/show_bug.cgi?id=723849
souphttp*: add ability to do HTTP session logging

This changeset adds the loggin infrastructure and
mods both souphttpsrc and souphttclientsink to use it.

https://bugzilla.gnome.org/show_bug.cgi?id=721764