[macos] Use gesture recognizer for context menu

NSView’s built-in context menu handling preempts non-right-click gesture recognizers, so preempt it with a right-click gesture recognizer that does essentially the same thing. With this change, two-finger double-tapping on a trackpad also zooms out, consistent with MapKit.

Fixes #5078.

platform/macos/src/MGLMapView.mm

@@ -395,6 +395,10 @@ public:
     clickGestureRecognizer.delaysPrimaryMouseButtonEvents = NO;
     [self addGestureRecognizer:clickGestureRecognizer];
     
+    NSClickGestureRecognizer *rightClickGestureRecognizer = [[NSClickGestureRecognizer alloc] initWithTarget:self action:@selector(handleRightClickGesture:)];
+    rightClickGestureRecognizer.buttonMask = 0x2;
+    [self addGestureRecognizer:rightClickGestureRecognizer];
+    
     NSClickGestureRecognizer *doubleClickGestureRecognizer = [[NSClickGestureRecognizer alloc] initWithTarget:self action:@selector(handleDoubleClickGesture:)];
     doubleClickGestureRecognizer.numberOfClicksRequired = 2;
     doubleClickGestureRecognizer.delaysPrimaryMouseButtonEvents = NO;
@@ -1359,6 +1363,14 @@ public:
     }
 }
 
+/// Right-click to show the context menu.
+- (void)handleRightClickGesture:(NSClickGestureRecognizer *)gestureRecognizer {
+    NSMenu *menu = [self menuForEvent:[NSApp currentEvent]];
+    if (menu) {
+        [NSMenu popUpContextMenu:menu withEvent:[NSApp currentEvent] forView:self];
+    }
+}
+
 /// Double-click or double-tap to zoom in.
 - (void)handleDoubleClickGesture:(NSClickGestureRecognizer *)gestureRecognizer {
     if (!self.zoomEnabled || gestureRecognizer.state != NSGestureRecognizerStateEnded