vout: ios: fix gpu_ReturnNotPermittedKillClient crashes
cf. https://developer.apple.com/library/content/qa/qa1766/_index.html No OpenGLES call should be made once the application entered background. To fix this issue, this commit disable any rendering (_appActive = NO;) and flush all OpenGLES commands from the applicationDidEnterBackground notification. This commits adds a lock and a condition in order to let the main thread know when the VLC vout thread unlocked the EAGLContext (because we don't want to flush this context while it's being used by the vout thread).
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