In order to understand memory layout consider the declaration of the
following structs. The first one is a part of our API:

struct vpx_codec_ctx {
  // ...
  struct vpx_codec_priv *priv;
};

The second one is defined in vpx_codec_internal.h:

struct vpx_codec_priv {
  // ...
};

The following struct is defined 4 times for encoder/decoder VP8/VP9:

struct vpx_codec_alg_priv {
  struct vpx_codec_priv base;
  // ... 
};

Private data allocation for the given ctx:

struct vpx_codec_ctx *ctx = <get>
struct vpx_codec_alg_priv *alg_priv = <allocate>
ctx->priv = (struct vpx_codec_priv *)alg_priv;

The cast works because vpx_codec_alg_priv has a
vpx_codec_priv instance as a first member 'base'.

Change-Id: I10d1afc8c9a7dfda50baade8c7b0296678bdb0d0