avcodec/snow: make new_picture const

Fixes "assignment discards const qualifier from pointer target type"

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

libavcodec/snow.h

@@ -117,7 +117,7 @@ typedef struct SnowContext{
     H264QpelContext h264qpel;
     MpegvideoEncDSPContext mpvencdsp;
     SnowDWTContext dwt;
-    AVFrame *new_picture;
+    const AVFrame *new_picture;
     AVFrame *input_picture;              ///< new_picture with the internal linesizes
     AVFrame *current_picture;
     AVFrame *last_picture[MAX_REF_FRAMES];