h264: support frame size changes with multi threading

Based on code by Janne Grunau
Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/h264.h

@@ -452,6 +452,8 @@ typedef struct H264Context {
     int nal_length_size;  ///< Number of bytes used for nal length (1, 2 or 4)
     int got_first;        ///< this flag is != 0 if we've parsed a frame
 
+    int context_reinitialized;
+
     SPS *sps_buffers[MAX_SPS_COUNT];
     PPS *pps_buffers[MAX_PPS_COUNT];