Fix tabs drawing over rest of MacVim in macOS 14 Sonoma

macOS 14 now defaults clipsToBounds to false, which works in most places
in MacVim but seems to cause issues with PSMTabBar which relies on
clipping to work. Since we are replacing it soon, simply set
clipsToBounds to true for the control.

Fix #1439

src/MacVim/PSMTabBarControl/source/PSMTabBarControl.m

@@ -147,6 +147,10 @@ - (void)initAddedProperties
         }
         [_addTabButton setNeedsDisplay:YES];
     }
+
+#if MAC_OS_X_VERSION_MAX_ALLOWED >= 140000
+    [self setClipsToBounds:YES];
+#endif
 }
 
 - (id)initWithFrame:(NSRect)frame