NSBox: (re-)implement isOpaque (#254)

This (re-)implements logic where a box is marked
as opaque if it is a custom box which is not
transparent, but moves the logic to `GSTheme`,
allowing themes to override this loggic.

This fixes the rendering of custom boxes with a
custom fill color; the current implementation
would mark them as non-opaque and hence the fill
color would not be rendered.

- The original implementation (from 1999) always
  returned `YES`
  (53bcb50240)
- A more complete implementation was backported
  from the Testplant branch in 2012
  (e85b16bc05, by
  @fredkiefer, upstream commit
  36e77b95f7)
- More recently, the a return value of `NO` was
  hardcoded
  (02bc49e2d5, by
  @ericwa)

Source/GSThemeDrawing.m

@@ -3475,6 +3475,18 @@ static NSDictionary *titleTextAttributes[3] = {nil, nil, nil};
     }
 }
 
+- (BOOL) isBoxOpaque: (NSBox *)box
+{
+  if ([box boxType] == NSBoxCustom)
+    {
+      return ![box isTransparent];
+    }
+  else
+    {
+      return YES;
+    }
+}
+
 - (void) drawBoxInClipRect: (NSRect)clipRect
 		   boxType: (NSBoxType)boxType
 		borderType: (NSBorderType)borderType