Fix copying of MEMBUFs to MEMBUFs. This case was implemented by using
the same code as the VIDBUF8 case, so it only worked for depths <= 8.
The 2 directions for copying between VIDBUFs and MEMBUFs worked by using
a Read/Write organization which makes the destination a VIDBUF so the
MEMBUF case was not reached, and the VIDBUF cases have already been fixed.

Fix this by removing "optimizations" for the VIDBUF8 case so that the
MEMBUF case can fall through to the general (non-segmented) case. The
optimizations were to duplicate code for the VIDBUF8 case so as to
avoid 2 multiplications by 1 at runtime. This optimization is not useful
since the multiplications are not in the inner loop.

Remove the same "optimization" for the VIDBUF8S case. It was even less
useful there since it duplicated more to do relatively less.