lavu: keep context size variables

They are essential to be able to use the utils without av_malloc()
That is for example use with malloc(), memalign(), some other
private allocation function, on the stack or others.

Signed-off-by: Michael Niedermayer <michaelni@gmx.at>

doc/APIchanges

@@ -114,7 +114,6 @@ API changes, most recent first:
 
 2012-10-xx - xxxxxxx - lavu 51.43.0 - aes.h, md5.h, sha.h, tree.h
   Add functions for allocating the opaque contexts for the algorithms,
-  deprecate the context size variables.
 
 2012-10-xx - xxxxxxx - lavf 54.18.0 - avio.h
   Add avio_closep to complement avio_close.