lavc: deprecate the use of AVCodecContext.time_base for decoding

When decoding, this field holds the inverse of the framerate that can be
written in the headers for some codecs. Using a field called 'time_base'
for this is very misleading, as there are no timestamps associated with
it. Furthermore, this field is used for a very different purpose during
encoding.

Add a new field, called 'framerate', to replace the use of time_base for
decoding.

doc/APIchanges

@@ -13,6 +13,10 @@ libavutil:     2014-08-09
 
 API changes, most recent first:
 
+2014-10-xx - xxxxxxx - lavc 56.5.0 - avcodec.h
+  Replace AVCodecContext.time_base used for decoding
+  with AVCodecContext.framerate.
+
 2014-10-xx - xxxxxxx - lavc 56.4.0 - avcodec.h
   Add AV_HWACCEL_FLAG_IGNORE_LEVEL flag to av_vdpau_bind_context().