Optimize painting for rectangular windows
At the moment, a window being painted is clipped to its region to paint properly non-rectangular windows. But this is not necessary for rectangular only windows. Moreover, clipping is currently badly implemented as it's applied on the buffer picture instead of the window picture itself.
Optimize painting of rectangular windows (closes #23).
Clip non-rectangular Window Picture to their proper Region, but do nothing for
rectangular Windows are this is already done by RenderComposite. This also
cleans up a dirty hack where the buffer Picture was clipped to the Window
Region twice before painting a window.