Disposing Pens and Brushes

[ddTech]

I don't know if is necessary when handling gdi+ through the gdPlusX-wrapper but I remember when I dealt with gdi+ functions directly it was crucial to dispose pens and brushes in order to avoid memory leaks.
As these methods are exposed in gdiPlusX I think we should use them and if just for good manners.
I'm doing this not only in the .Destroy() but also in _.prepareBrushes() where pens or brushes are being overwritten by new ones.

In this context I changed initialization of the _o - properties from (none) or .F. to .NULL.. This way isNull()-tests do not necessarily require an additional type() test. ... and it's just "more correct".

Regards

Frank