Add a basic custom painting facility to Sky

sky/engine/core/rendering/RenderBox.cpp

Index: sky/engine/core/rendering/RenderBox.cpp
diff --git a/sky/engine/core/rendering/RenderBox.cpp b/sky/engine/core/rendering/RenderBox.cpp
index c0ab2dc08236972ce6087cf7622169f9db579371..c986599473bb56798dfbda9d6f1f6b0e7837d897 100644
--- a/sky/engine/core/rendering/RenderBox.cpp
+++ b/sky/engine/core/rendering/RenderBox.cpp
@@ -989,6 +989,12 @@ void RenderBox::paintBoxDecorationBackground(PaintInfo& paintInfo, const LayoutP
     LayoutRect paintRect = borderBoxRect();
     paintRect.moveBy(paintOffset);
     paintBoxDecorationBackgroundWithRect(paintInfo, paintOffset, paintRect);
+    // TODO(abarth): Currently we only draw m_customPainting if we happen to

[ojan, 2015/03/19 22:30:04]

I think we want to do this before we do any other painting. I think in the final
result we'll probably want 3 different callbacks. beforePaint, paint and
afterPaint.

beforePaint: happens before the default paint (including backgrounds, etc.
paint: the default paint. if you set this callback then we skip the default
paint code.
afterPaint: happens after paint. :)

We can fix this in followup patches of course.

[abarth-chromium, 2015/03/19 22:34:27]

Yeah, the main design question is whether these paint callbacks should be
synchronous.  One idea is to make them asynchronous (as implemented in this CL),
and basically generate SkPictures that get played back at these times.  The
script is then responsible for invalidating the SkPictures as appropriate. 
Presumably we'll want to make it easy to invalidate your picture when the size
of your box changes.

+    // have a box decoration or a background. Instead, we should probably have
+    // another function like paintBoxDecorationBackground that subclasses can
+    // call to draw m_customPainting.
+    if (m_customPainting)
+        paintInfo.context->drawDisplayList(m_customPainting.get(), paintRect.location());
 }
 
 void RenderBox::paintBoxDecorationBackgroundWithRect(PaintInfo& paintInfo, const LayoutPoint& paintOffset, const LayoutRect& paintRect)