vp9: move VP9SharedContext back to the top of VP9Context

VP9SharedContext needs to be the first member so its properties can be
safely accessed from hardware accelerators, without the need to share
the full VP9Context.

Fixes ticket #6674.

libavcodec/vp9dec.h

@@ -89,8 +89,8 @@ typedef struct VP9Block {
 typedef struct VP9TileData VP9TileData;
 
 typedef struct VP9Context {
-    VP9TileData *td;
     VP9SharedContext s;
+    VP9TileData *td;
 
     VP9DSPContext dsp;
     VideoDSPContext vdsp;