Set a non-zero default cacheBuffer.

Set a non-zero default cacheBuffer.

The likelihood of constant framerate when flicking is considerably
improved by having a cacheBuffer.  Since there is only a minimal cost
in having a modest cacheBuffer, it is better to have a more optimal
default value - 320 seems a good starting point.

Change-Id: Id3c8a153821573b5b08c6fbd80d34152908d358d
Reviewed-by: Bea Lam <bea.lam@nokia.com>