Fixing tiles X-mode bug where some out of sight, but previously squares were incorrectly showing up all black.

git-svn-id: https://crawl-ref.svn.sourceforge.net/svnroot/crawl-ref/trunk@6979 c06c8d41-db1a-0410-9941-cceddc491573

Created by  ennewalker  on September 24, 2008
6MQLXDOSZALJUJZ4WAIXX3V4FF245UGTDOIXURY3FCY3WAL7FDVQC
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