Add planar audio support, improve test output

- Add planar audio support.  FFmpeg and libav use planar audio for many
  encoders, so it was somewhat necessary to add support in libobs
  itself.

- Improve/adjust FFmpeg test output plugin.  The exports were somewhat
  messed up (making me rethink how exports should be done).  Not yet
  functional; it handles video properly, but it still does not handle
  audio properly.

- Improve planar video code.  The planar video code was not properly
  accounting for row sizes for each plane.  Specifying row sizes for
  each plane has now been added.  This will also make it more compatible
  with FFmpeg/libav.

- Fixed a bug where callbacks wouldn't create properly in audio-io and
  video-io code.

- Implement 'blogva' function to allow for va_list usage with libobs
  logging.

libobs/media-io/video-io.h

@@ -25,6 +25,8 @@ extern "C" {
 
 /* Base video output component.  Use this to create an video output track. */
 
+#define MAX_VIDEO_PLANES 8
+
 struct video_output;
 typedef struct video_output *video_t;
 
@@ -49,8 +51,8 @@ enum video_format {
 };
 
 struct video_frame {
-	const void        *data;
-	uint32_t          row_size; /* for RGB/BGR formats and UYVX */
+	const uint8_t     *data[MAX_VIDEO_PLANES];
+	uint32_t          row_size[MAX_VIDEO_PLANES];
 	uint64_t          timestamp;
 };