Multiple Starling instances fix and drawRect bug

Fixes for https://github.com/StarlingGraphics/Starling-Extension-Graphics/issues/63
and https://github.com/StarlingGraphics/Starling-Extension-Graphics/issues/64

Allowing for multiple Starling instances to work with Graphics API. The Starling instance that
draws and creates the Graphic needs to be made current by a call to makeCurrent().

In a multiple Starling scenario, the programmer needs to keep track of each Starling instance himself to 
make each instance current as needed.

drawRect did not use _strokeThickness to complete the rect, leaving the top left corner missing _strokeThickness/2 in height