fixed wrong BitmapData size in 'drawToBitmapData' when using HiDPI display (closes #618)

Starling now contains properties for both "backBufferWidth/Height" and "viewPortWidth/Height";
one returns the actual size in pixels, the other in points. This is necessary because Stage3D is inconsisent with its back buffer dimensions. Some methods (e.g. 'setScissorRectangle') use points, others (e.g. 'backBufferWidth/Height') use pixels.