Fix billboard error which used the wrong width for top and bottom width of trapezoid billboards. The reason is the use of cross-product, which flips the vertical axis used in calculations. Properly documented in the code now, API now works as expected. Bug and fix provided by pc0de.

git-svn-id: svn://svn.code.sf.net/p/irrlicht/code/trunk@4030 dfc29bdd-3216-0410-991c-e03cc46cb475