Switch to instance methods of draw and display

This patch switches to instance methods of draw and display for
ASTextNode and ASImageNode to attempt to increase their performance.

It also fixes some thread safety issues in ASImageNode which appear
to have been regressions (though probably not hit very often).

And it sets up work for allowing modification of CGContexts before
and after a node's contents are drawn.

AsyncDisplayKit/ASDisplayNode.h

@@ -37,6 +37,11 @@ typedef CALayer * _Nonnull(^ASDisplayNodeLayerBlock)();
  */
 typedef void (^ASDisplayNodeDidLoadBlock)(ASDisplayNode * _Nonnull node);
 
+/**
+ * ASDisplayNode will / did render node content in context.
+ */
+typedef void (^ASDisplayNodeContextModifier)(CGContextRef context);
+
 /**
  Interface state is available on ASDisplayNode and ASViewController, and
  allows checking whether a node is in an interface situation where it is prudent to trigger certain