Fix some race conditions in mlt_consumer.

OpenGL apps need to receive consumer-thread-stopped *after* all of the
frames are closed. Otherwise, it may cleanup the GL context before
frames holding context resources are closed.
Some consumer threads call mlt_consumer_purge().
Movit and glew pkg-config have 'GL' in includes.
Give Qt includes highest priority.

This gives a non-system-installed version of Qt specified via QTDIR
priority over the system installed version of Qt.
Fix regression in consumer sdl fullscreen=1.

Caused by commit 0f587b2.
Add a reference to the consumer on the frame.

WebVfx uses this to learn about the consumer so it can listen to the
consumer-stopping event to prevent a deadlock.
Fix regression in commit f26c43f.
Do not try to use open_mutex unless it has been init.
Always init pthread mutexes regardless if locking.
Fix possible divide by zero.

Discovered using auto-profile with test file "NAO Kinect control.wmv"
Make xml non-seekable only if explicitly set.
Fix serializing an xml producer by itself.

Applications should use the xml producer's _original_type and
_original_resource properties and coerce it to a playlist or tractor to
serialize the entire graph of nodes.
Fix crash when LADSPA_PATH is empty string.
Add 'boolean' and 'argument' to service metadata schema.
Add filter_panner.yml metadata.
frame leak fix
Fix memory leak converting image in RAM to GL texture.
Fix crash removing filter attached to a service.

There can still be frame objects that have a filter's get_image function
in its image processing stack. Need to add a reference to the filter on
the frame objects.
Frei0r glow is not thread-safe.
race condition fix
Fix videostab2 interpolation.

This filter uses RGB mode, for which bicubic is broken. vid.stab still
to this day uses bilinear with packed pixel formats. In order to fix
bilinear, needed to remove extra calls to floor function.