Now we use a ForcedLayer in the navigator overlay to cache the recording for
each entry in the overlay. This mechanism just caches the display list, not the
underlying pixels.
Also, remove the "dispose" notification in the Layer tree because it was
disposing layer too eagerly. We don't actually need this notification for
anything other than eagerly freeing some C++ memory.
This widget is slow and not commonly used. The original reason we added it was
to colorize images, but now that feature is built into the image and background
image widgets.
Fixes#1646
Code outside of package:sky should import this code using
package:sky/rendering.dart
package:sky/widgets.dart
Moving this code into the "src" directory is a convention that signifies that
and it cleans up the generated dartdoc because the libraries in the src
directory aren't included in the generated documentation. Instead, the classes
are documented in the widgets.dart and rendering.dart libraries.
