bug#21662: 25.0.50; visible-bell causes display artifacts on OS X 10.11

[Mustafa Kocaturk <murskt@gmail.com>]

Hi,

The current implementation of the visual bell on NextStep is to cache and redraw a rectangle on the current Emacs frame *from outside the NextStep run loop*. It seems this was good enough so far but is no longer a particularly supported one for the latest Mac OS X version.
Maintaining it might still be possible, only with sufficient effort and caution.

Alternative: Switch to an implementation that is particularly supported in this release of OS X, as follows:
Create a temporary NSView object and attach it to the selected EmacsView object.
This new subview draws itself, starts an NSTimer, with a callback in the current NS run loop. 
When the timer expires the callback is entered, which updates, hides, and finally removes the subview from its superview.
The view is garbage collected, all as part of the NS run loop. 
The required change is small: Add a NSView-derived class in nsterm.m and replace old cache/draw/restore code with new temporary local instance of this new class.

Best regards,
Mustafa