OS X: set scroller knob style to light.

On newer OS X versions the scrollbar is overlaid on the window
contents, so a black know is invisible.

src/platform/cocoamain.mm

@@ -1015,6 +1015,7 @@ void InitTextWindow() {
     [TW setContentView:scrollView];
     [scrollView setBackgroundColor:[NSColor blackColor]];
     [scrollView setHasVerticalScroller:YES];
+    [scrollView setScrollerKnobStyle:NSScrollerKnobStyleLight];
     [[scrollView contentView] setCopiesOnScroll:YES];
 
     TWView = [[TextWindowView alloc] init];