propagate user private data on decode

The pointer passed in the user_priv argument to vpx_codec_decode()
should be propagated through to the corresponding output frame and
made available in the image's user_priv member. Fixes issue #252

Change-Id: I182746a6882c8549fb146b4a4fdb64f1789eb750

vp8/vp8_dx_iface.c

@@ -466,6 +466,7 @@ static vpx_codec_err_t vp8_decode(vpx_codec_alg_priv_t  *ctx,
             vpx_img_set_rect(&ctx->img,
                              VP8BORDERINPIXELS, VP8BORDERINPIXELS,
                              sd.y_width, sd.y_height);
+            ctx->img.user_priv = user_priv;
             ctx->img_avail = 1;
 
         }