macosx: Add tickmark drawing to VLCHUDSliderCell

The Slider cell was using the tick mark drawing that the OS provided,
which could cause them to be drawn with dark color on dark background
on older OS X versions.
This commit adds the missing tick mark drawing.

modules/gui/macosx/VLCHUDSliderCell.m

@@ -175,4 +175,17 @@ NSAffineTransform* RotationTransform(const CGFloat angle, const NSPoint point)
     [path fill];
 }
 
+- (void)drawTickMarks
+{
+    for (int i = 0; i < self.numberOfTickMarks; i++) {
+        NSRect tickMarkRect = [self rectOfTickMarkAtIndex:i];
+        if (self.isEnabled) {
+            [_strokeColor setFill];
+        } else {
+            [_disabledStrokeColor setFill];
+        }
+        NSRectFill(tickMarkRect);
+    }
+}
+
 @end