modalscreens now resize to fit parent.
updated meshviewer example to lock the image to the bottom left corner.
tooltips:
added EGDC_TOOLTIP_BACKGROUND to skins.
changed the position of tooltips and their time between creation to 500ms.
made sure that they do not leave the area of the gui environment.
new method: rect::constrainTo to forcing one rectangle to stay inside another.
git-svn-id: http://svn.code.sf.net/p/irrlicht/code/trunk@697 dfc29bdd-3216-0410-991c-e03cc46cb475
