Fix 8bit grey image TGAs, which were not working due to missing palette. Also switched to RGB8 format, as otherwise a loss in precision would occur. Thanks to Klunk for the error report and a test image.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4160 dfc29bdd-3216-0410-991c-e03cc46cb475