Disable the on disk pixmap cache (affects icon caching) as this causes crashes for some use cases, as
discussed on bug 160284. There are two known causes:

1. Alignment of mmap()'ed data is off, which on some architectures results in a SIGBUS.
2. We can also receive SIGBUS for trying to access mmap()'ed pages where the underlying file has been
truncated, which has also been an issue.

I'm working on making the code properly handle the SIGBUS signal which we can receive if our mmap()'ed
cache is deleted from under us. I will also fix a separate issue with alignment of the mmap()'ed data if
I can fix the SIGBUS issue.