avcodec/svq3: Avoid overhead of AVBuffer API

avcodec/svq3: Avoid overhead of AVBuffer API

Up until now, the SVQ3 decoder allocated several refcounted buffers,
despite no sharing/refcounting happening at all: Their refcount never
exceeds one and they are treated like ordinary buffers (with the
exception that the pointer used to access them is in the middle of the
allocated buffer, but this does not warrant using the AVBuffer API at
all). Given that using the AVBuffer API incurs overhead, it is no longer
used at all.

Signed-off-by: Andreas Rheinhardt <andreas.rheinhardt@gmail.com>