Fix for Issue# 63 - strokeThickness not used

See
https://github.com/StarlingGraphics/Starling-Extension-Graphics/issues/63

Using drawRect didn't result in a fully closed rectangle
Multiple Starling instances failure

https://github.com/StarlingGraphics/Starling-Extension-Graphics/issues/64

When multiple Starling instances are used, the Graphic extension
graphics didnt show up on the second Starling instance. Problem was with
cached shaders in Graphic.as