linkmap: Fix for 3.20, where GtkDrawingArea doesn't draw the background

According to bug #761341, this is the expected fix.

Author: kaiw

meld/linkmap.py

@@ -64,6 +64,10 @@ def do_draw(self, context):
         context.rectangle(0, clip_y, allocation.width, clip_height)
         context.clip()
 
+        stylecontext = self.get_style_context()
+        Gtk.render_background(
+            stylecontext, context, 0, clip_y, allocation.width, clip_height)
+
         height = allocation.height
         visible = [self.views[0].get_line_num_for_y(pix_start[0]),
                    self.views[0].get_line_num_for_y(pix_start[0] + height),