Seeking on a block boundary that is exactly the end of our already
cached data was incorrect.

Define the new sout_EncoderCreate in missing.c
Signed-off-by: Rémi Denis-Courmont <rdenis@simphalempin.com>
with little modifications

Perhaps we should turn it into ...New() and do more than just allocate
the object.

(You get to love va_list type-safety)