Improve multi instance support for osx desktop capture

Fixes behavior with multiple desktop captures in the same scene

Also create the dispatch_queue without a name to prevent name collisions
Use recursive mutex for scene mutex

Fixes a deadlock when trying to remove a source from the GUI. The scene
item signal handlers would mark the source as removed which results in
the video thread also trying to run obs_sceneitem_destroy thereby
deadlocking the video thread (and the GUI thread)
Check if scene item is actually alive when calling obs_sceneitem_destroy

Prevents a double free in case the scene item was previously removed
Lock graphics before trying to remove a scene item

Prevents another deadlock if removing the scene item triggers source
cleanup of a source with graphics resources
Fix obs_sceneitem_destroy for scenes with more than one scene item
Fix the design flaw with obs_sceneitem_destroy

The previous commit used the scene as a  parameter to check to see if
the scene item was still present within the scene before destroying, but
this was actually unnecessary because the fault was because the destroy
signal was being triggered *before* the scene's mutex locked, thus
causing a race condition.  I changed the code so that it signals after
the lock instead of before, so the scene parameter should no longer be
necessary.
Merge branch 'master' of https://github.com/jp9000/obs-studio
Improve thread safety for scene items

Scene items previously were removed by calling obs_sceneitem_destroy,
but this proved to be a potential race condition where two different
threads could try to destroy the same scene item at the same time.

Instead of doing that, reference counting is now used on scene items,
and an explicit obs_sceneitem_remove function is used instead for item
removal, which sets a 'removed' variable to ensure it can only be called
exactly one time.
Use mutex instead of variable for RC check
Clean up the previous commit a bit

The was_removed variable was rather unnecessary
Oh and while we're at it I suppose another fix
And cover the one in scene_destroy
Fix another bug (don't commit without compiling)

Also don't code in anger
Merge pull request #35 from BtbN/master

git describe based version number support for CPack