Fix toggling native fullscreen on/off to restore window to correct size

Previously when using native fullscreen mode, if you toggle fullscreen
off, the window will end up taking up the whole screen rather than
restoring back to the original size. Fix that.

The real issue is because when you resize MacVim's window (which the
fullscreen restore code does), then callback (windowDidResize) triggered
a complicated set of callbacks by calling setFrameSize:, which in turn
resizes Vim, which in turn calls windowDidResize again, which usually
does the right thing, but not always. Fix the window resize handler code
to always respect the new window size by calling
setFrameSizeKeepGUISize: instead which doesn't resize the window.

Author: ychin

src/MacVim/MMWindowController.m

@@ -1047,14 +1047,10 @@ - (void)windowDidResize:(id)sender
     // may resize automatically) we simply set the view to fill the entire
     // window.  The vim view takes care of notifying Vim if the number of
     // (rows,columns) changed.
-    if (shouldKeepGUISize) {
-        // This happens when code manually call setFrame: when we are performing
-        // an operation that wants to preserve GUI size (e.g. in updateToolbar:).
-        // Respect the wish, and pass that along.
-        [vimView setFrameSizeKeepGUISize:[self contentSize]];
-    } else {
-        [vimView setFrameSize:[self contentSize]];
-    }
+    // Calling setFrameSizeKeepGUISize: instead of setFrameSize: prevents a
+    // degenerate case where frameSizeMayHaveChanged: ends up resizing the window
+    // *again* causing windowDidResize: to be called.
+    [vimView setFrameSizeKeepGUISize:[self contentSize]];
 }
 
 - (void)windowDidChangeBackingProperties:(NSNotification *)notification