introduce clipping

Created by: otsakir

Have a global clipping module that will contain the a transparent "rectangle" . It will be the whole screen by default. Before blitting a Renderable the clipping module wil be set accordingly. The Renderable itself will take into account the clipping module when blitted. Thus:

• if it falls outside the visible area won't be blitted at all
• if it is partially hidden it should half displayed