Screen/OSX: Fix window placement and position/size queries under Cocoa.

On multi-display setups, fullscreen windows on secondary displays were
misplaced if use of Cocoa was requested for fullscreen mode. -> Fixed.
Related, Screen('Rect') and Screen('GlobalRect') could return wrong
info on windows placed on secondary displays if the windows were
kPsychGUIWindows's. -> Fixed.
Screen/OSX: Allow OpenGL context sharing between fullscreen and windowed windows.

Both the Cocoa window setup code and the pure fullscreen setup code now
share the same underlying CGL contexts. For Cocoa, NSOpenGL contexts
are wrapped around the CGL contexts. The fact windowed vs. non-windowed
onscreen windows nor always operate with OpenGL contexts from CGL
allows OpenGL context resource sharing across fullscreen CGL and
windowed Cocoa windows.

All that was needed was to remove the pixelformat attribute
kCGLPFAFullScreen. This attribute is deprecated as of OSX 10.6, not
really needed anymore on 10.6+, and it was preventing context resource
sharing.

-> Testing on OSX 10.7.5 with NVidia GeForce 330M on dual-display
worked.
-> Screen's for OSX rebuilt.