New: + CPColor checkerBoardColor for debug use.

This color has a standard 'transparency' checkerboard pattern which is especially useful for our scrolling tests where it makes scrolling easy to visualise.

Refs #2078, #2079.

AppKit/CPColor.j

@@ -813,6 +813,12 @@ url("data:image/png;base64,BASE64ENCODEDDATA")  // if there is a pattern image
     return [CPColor colorWithRed:RAND() green:RAND() blue:RAND() alpha:1.0];
 }
 
++ (CPColor)checkerBoardColor
+{
+    // Thanks to cocco http://stackoverflow.com/a/18368212/76900.
+    return [CPColor colorWithPatternImage:[[CPImage alloc] initWithContentsOfFile:"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAABAAAAAQAQMAAAAlPW0iAAAABlBMVEX////MzMw46qqDAAAAEElEQVQImWNg+M+AFeEQBgB+vw/xfUUZkgAAAABJRU5ErkJggg=="]];
+}
+
 @end
 
 /// @cond IGNORE

Tests/Manual/CPScrollView/AppController.j

@@ -59,6 +59,7 @@
     var scrollView = [self makeScrollview];
     [scrollView setFrameOrigin:CGPointMake(20,20)]
 
+    [[scrollView documentView] setBackgroundColor:[CPColor checkerBoardColor]];
     [[scrollView documentView] addSubview:innerScrollView];
     [[aWindow contentView] addSubview:scrollView];
 

Tests/Manual/CPScrollView2/AppController.j

@@ -31,6 +31,7 @@
 
     [scrollView setDocumentView:contentView];
     [contentView2 setAutoresizingMask:CPViewWidthSizable];
+    [contentView2 setBackgroundColor:[CPColor checkerBoardColor]];
     [scrollView2 setDocumentView:contentView2];
 }