tsdemux: implement proper seeking for h264 streams.

Co-Authored by: Thibault Saunier <tsaunier@gnome.org>

From a high level perspective, the new process for seeking h264
streams is as follows:

1) Rewind the stream until we find the first I-slice of a frame,
   and mark its offset in the stream.
2) Rewind the stream until we find SPS and PPS informations,
   to make sure the subsequent parser is up to date.
3) Accumulate optionnal SEI NAL units on the way.
4) Push the SPS, PPS and SEI units before the new keyframe.

https://bugzilla.gnome.org/show_bug.cgi?id=675132
gl/examples: update for TEXTURE_2D change
glfilterapp: actually emit the client-draw signal
docs: update for some gl function name changes
docs: add gluploadmeta object
docs: misc adds for gl functions
docs: add glframebuffer object
docs: add gsteglimagememory
mpegtspacketizer: Fix wraparound detection issue

Using 32bit unsigned values for corrected pcr/offset meant that we
potentially ended up in bogus values

Furthermore, refpcr - refpcroffset could end up being negative, which
PCRTIME_TO_GSTTIME() can't handle (and returned a massive positive value)
mpegtspacketizer: live: handle pcr/dts discrepancies some more

When wrapover/reset occur, we end up with a small window of time where
the PTS/DTS will still be using the previous/next time-range.

In order not to return bogus values, return GST_CLOCK_TIME_NONE if the
PTS/DTS value to convert differs by more than 15s against the last seen
PCR

https://bugzilla.gnome.org/show_bug.cgi?id=674536
glmixer: allow the subclass to choose the sink pad type

Allows custom properties to be placed on the sink pads by subclasses
glvideomixer: add positioning of input streams

https://bugzilla.gnome.org/show_bug.cgi?id=729798
glvideomixer: support input frame scaling
glvideomixer: wire up the alpha pad property
gldownload: plug a memory leak
glvideomixer: silence incorrect number of arguments in format warning
tsdemux: Don't overwrite original seek value

In accurate mode, we'll be using start to fill in the seek segment,
therefore don't overwrite it with the shifted seek position.

https://bugzilla.gnome.org/show_bug.cgi?id=731698
opengl: duplicate glfiltercube class and rename it
opengl: add new plugin to build
opengl: init transformation plugin