d3dvideosink: Open Direct3D devices in a threadsafe way

Otherwise we'll get crashes when using the device from multiple
threads, e.g. when using multiple sinks at once.

https://bugzilla.gnome.org/show_bug.cgi?id=707523
po: update POTFILES

https://bugzilla.gnome.org/show_bug.cgi?id=726555
gl: Always include EGL_CFLAGS in GL_CFLAGS when EGL_LIBS is added to GL_LIBS

They should be handled in tandem, in case any EGL provider could require some
CFLAGS and set them (possibly once moved to prefer pkg-config files),
such as for a custom header location.
gl: Include GL_CFLAGS while checking for GL typedefs

Otherwise those checks may fail at configure time if they contain extra
include paths, while at build time they are included, potentially causing
incompatible typedefs between system GL headers and gstreamer compatibility
prototypes.

https://bugzilla.gnome.org/show_bug.cgi?id=733248
mxfmux: Avoid taking unnecessary ref

Fixes https://bugzilla.gnome.org/show_bug.cgi?id=733123
aggregator: Handle event seqnum
aggregator: Do not forget to reset the flow return when stoping

Setting it to FLUSHING when the element is not started, and to OK
when it starts.
aggregator: Flush sinkpads when stopping

All values are meaningless in that case, so we should make sure that
we clean everything
aggregator: Reset flow_return *after* stopping the srcpad task.

Otherwise it might be set in an already running aggregate function.
openslessink: Silence some error debug output to log output

These are not really errors, just unsupported features we don't
necessarily need.
openslesringbuffer: Provide the size of our array to GetDestinationOutputDeviceIDs

Otherwise it does not and just fails. It needs to know the size of the
array to not write too much to it.
watchdog: Add unit test

And improve some debug prints