Fix background filling

The split view's not drawing background (from 72fa298) should be
enough to take care of the glitch. We don't need to fill the
split view's background.

Fix #138.

MacDown/Code/Document/MPDocument.m

@@ -1305,16 +1305,11 @@ - (void)setupEditor
         [self.editor setValue:value forKey:key];
     }
 
-    CGColorRef backgroundCGColor = self.editor.backgroundColor.CGColor;
     NSView *editorChrome = self.editor.enclosingScrollView.superview;
-
     CALayer *layer = [CALayer layer];
-    layer.backgroundColor = backgroundCGColor;
+    layer.backgroundColor = self.editor.backgroundColor.CGColor;
     editorChrome.layer = layer;
-
-    layer = [CALayer layer];
-    layer.backgroundColor = backgroundCGColor;
-    self.splitView.layer = layer;
+    editorChrome.wantsLayer = YES;
 
     [self.highlighter activate];
     self.editor.automaticLinkDetectionEnabled = NO;