macosx: disable pending reshape mechanism

The _pendingReshape mechanism was used to avoid redrawing the view if it
has been updated externally, and avoid re-render the view when it's
being drawn already but:
 - `drawRect:` won't be called if we didn't change the window, except
   the first time which will likely trigger a clear.
 - When the window state changes, we were already setting
   _pendingReshape to YES.

Overall, we can let the UI system decide when it requires a re-draw and
react to that instead of trying to workaround when to re-draw.