Fix the text scene node placing in viewports. The problem was, that the calculation took the absolute viewport dimension into account, while rendering into the viewport requires the screenwidth to be taken. Hence, the text was usually misplaced. The collision method can still be used for this mixed mode calculation (get screen coordinate during viewport rendering, use it for full screen rendering), but requires an additional parameter.
Software drivers don't support 2d rendering into viewports, so they still fail to properly position text (or other 2d things) into a viewport.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@3489 dfc29bdd-3216-0410-991c-e03cc46cb475