Adding IVideoDriver::drawPixel() and implementations, as per http://irrlicht.sourceforge.net/phpBB2/viewtopic.php?t=30741. Test code is in that post.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@1642 dfc29bdd-3216-0410-991c-e03cc46cb475