Resolves: fdo#84729 buffer cache grows to over 9000 unused entries

regression from

commit a5351434fbea1126961f5dbaa986e7c9e686c24e
Author: tsahi glik <tsahi.glik@cloudon.com>
Date:   Thu Jan 16 12:51:44 2014 -0800
    fix rendering issues in iOS with aqua

because now the cache just fills up because nothing will have
a depth of "0" and a new entry is created every time.

0 could be seen to mean "whatever is a good non-1bit depth" or auto or
something. (Do the uses of "8" really mean 8)

0 passed to new VirtualDevice under Linux/X ends up using the depth of the
outputdev, so here set nBits to that value to start with and the problem
goes away

Change-Id: I4390d6f7edef9dc7430cc77e3518dc751a5f479d