Fixes an issue where a window's image would be stretched as it was moved further offscreen. The offscreen part of a window also did not display correctly in Exposé.
-- v2: winemac: Don't constrain surface dimensions to the onscreen part of a window. winemac: Set the Cocoa window contentMaxSize to the size limits from constrain_window_frame(). winemac: Refactor constrain_window_frame() to use separate origin and size arguments.