+ //! Returns true if the layer needs to be able to examine its context.
+ /*! context to render itself, other than for simple blending. For
+ ** example, the blur layer will return true - it can't do its job
+ ** if it can't see its context, and the circle layer will return
+ ** false - rendering a circle doesn't depend on the underlying
+ ** context until the final blend operation. */
+ virtual bool reads_context()const;
+