NSCursor updates in gui

[Richard Frith-Macdonald]

FYI I have changed the NSCursor hot point coordinates to be flipped as 
in MacOS-X, so you now need to specify the coordinates of the cursor 
hot point from the top left corner of the cursor image rather than 
from the bottom left corner.

I have also changed the gui to call -resetCursorRects whenever a view 
size is changed or when it is scrolled.  This change should fix all 
the glitches where cursors appear to change at the wrong positions in 
views, or where the cursor gets stuck with the wrong image.