Wrap FFmpeg operations in mutexes, switch to MP4

I can't believe I wasn't doing this.  This is why file output was
getting corrupted.  Audio and video send in data from separate threads.
I should be embarassed for not having considered that.

Key lesson:  Increase threading paranoia levels.  Apparently my
threading paranoid levels are lackluster.

obs/window-basic-main.cpp

@@ -601,7 +601,7 @@ void OBSBasic::on_recordButton_clicked()
 	} else {
 		QString path = QFileDialog::getSaveFileName(this,
 				"Please enter a file name", QString(),
-				"AVI Files (*.avi)");
+				"MP4 Files (*.mp4)");
 
 		if (path.isNull() || path.isEmpty())
 			return;