winemac: When ClipCursor() is called redundantly, don't warp the cursor or discard mouse move events.

Fixes a problem in some games which repeatedly (re)establish the same cursor
clipping rect, making it exceedingly difficult to move the camera with the
mouse.

In particular, keep the title bar from moving behind the menu bar or Dock (when
it's on the bottom).

user32: In WINPOS_SysCommandSizeMove(), fix constraining a point to a rect's right and bottom edges.

Ddraw and d3d9 already take care of this.