Moved Clear() implementation into wxDC using a new virtual CocoaGetBounds()

Moved Clear() implementation into wxDC using a new virtual CocoaGetBounds()
to determine the rect to clear.  Also added CocoaUnapplyTransformations()
to bring the coordinate system back into Cocoa coordinates for those
cases such as Clear() where it makes more sense.

git-svn-id: https://svn.wxwidgets.org/svn/wx/wxWidgets/trunk@31440 c3d73ce0-8a6f-49c7-b76d-6d57e0e08775