We can use a bit more RAM for graphics than we currently do, even on 32
bits, which should improve interactive performance as there will be less
swapping of bitmap data and re-parsing of SVGs.

Note that currently the value is effectively multiplied by 2, as the
limit is stored in GraphicCache::mnMaxDisplaySize, but there are 2
independent "counters" GraphicCache::mnUsedDisplaySize and
GraphicManager::mnUsedSize that are both checked against the same limit.

Change-Id: I4e33030af7dcd953c35672f80599188a1fbc4453