audio/video: merge decoder return values

Will be helpful for the coming filter support. I planned on merging
audio/video decoding, but this will have to wait a bit longer, so only
remove the duplicate status codes.

common/common.h

@@ -57,6 +57,13 @@ enum stream_type {
     STREAM_TYPE_COUNT,
 };
 
+enum {
+    DATA_OK     = 1,
+    DATA_WAIT   = 0,
+    DATA_AGAIN  = -1,
+    DATA_EOF    = -2,
+};
+
 extern const char *const mpv_version;
 extern const char *const mpv_builddate;