Add external constants.

Change-Id: I8d329513c87bd8622306aba27c83bf04ef170fa4

vpx/internal/vpx_codec_internal.h

@@ -238,8 +238,9 @@ typedef vpx_image_t *(*vpx_codec_get_frame_fn_t)(vpx_codec_alg_priv_t *ctx,
  *     using external frame buffers.
  *
  * \note
- * When decoding VP9, the application must pass in at least 8 external
- * frame buffers, as VP9 can have up to 8 reference frames.
+ * When decoding VP9, the application must pass in at least
+ * #VP9_MAXIMUM_REF_BUFFERS external frame buffers, as VP9 can have up to
+ * that many reference frames.
  */
 typedef vpx_codec_err_t (*vpx_codec_set_frame_buffers_fn_t)(
     vpx_codec_alg_priv_t *ctx,

vpx/vpx_decoder.h

@@ -362,8 +362,9 @@ extern "C" {
    *     using external frame buffers.
    *
    * \note
-   * When decoding VP9, the application must pass in at least 8 external
-   * frame buffers, as VP9 can have up to 8 reference frames.
+   * When decoding VP9, the application must pass in at least
+   * #VP9_MAXIMUM_REF_BUFFERS external frame buffers, as VP9 can have up to
+   * that many reference frames.
    */
   vpx_codec_err_t vpx_codec_set_frame_buffers(
       vpx_codec_ctx_t *ctx,

vpx/vpx_external_frame_buffer.h

@@ -17,6 +17,14 @@ extern "C" {
 
 #include "./vpx_integer.h"
 
+/*!\brief The maximum number of work buffers used by libvpx.
+ */
+#define VPX_MAXIMUM_WORK_BUFFERS 1
+
+/*!\brief The maximum number of reference buffers that a VP9 encoder may use.
+ */
+#define VP9_MAXIMUM_REF_BUFFERS 8
+
 /*!\brief External frame buffer
  *
  * This structure is used to hold external frame buffers passed into the