eglglessink: Don't set the EAGLContext of the main thread and don't reuse it either

... instead create a new context for every sink instance.

https://bugzilla.gnome.org/show_bug.cgi?id=720421
audiomixer: Also resync timestamps on the RESYNC flag
mssdemux: lock around 'cancelled' flag

Prevents race conditions when pipeline is seeking near eos
mssdemux: do not push event holding object lock

Can lead to deadlocks as the push might block downstream in
serialized event cases.
mssdemux: updating docs explaining how it works

mssdemux changed a lot and the docs were not correct anymore.

Also adds the 'Adaptive' category to its details
sndfile: rewrite sndfile for 1.0

Add a sfdec for a start. Instead of a source plugin, this is a demuxer/decoder combination. This makes it work with auto-plugging.
sfdec: add query implementation for position and duration
sfdec: add taglist support

Map the metadata strings and a bunch of info-fields to GStreamer tags.
sfdec: add event function and seeking support
sfdec: make _stop() non fatal when we have an error already

Having no open file in _stop() is okay. Just reset state.
modplug: small seeking code reshuffle

Put the code that checks stop values together. Don't clamp a cur value of -1 to 0 (should not happen right now).
applemedia: atdec: fix aac decoding on Mavericks
applemedia: vtdec: set the correct stride in the video meta