Libvpx was memseting every external frame buffer before decode. This
was to work around a valgrind issue in our C loop filter. Most of
the time this was not needed and we have noticed some significant
performance loss on some platforms. Now we require the application to
zero out the buffers if it is using external frame buffers.

Change-Id: I7330d00a315e65137ed30edd5f813e8929b76242