Provides read-write access to the render state.
The render state is represented by the render parameters,
portlet mode and window state.
The mutable render state is available through certain portlet response and
portlet URL interfaces.
When changed on one of these interfaces, the modified render state becomes
active according to the rules of the originating object.
If the render state is modified on a response, it becomes active for
subsequent portlet render phases.
If the render state is modified on a URL, it becomes active when the URL
containing the modified state is activated.
For example, the methods defined here can be used to set a new portlet mode and
render parameters on a
PortletURL object.
When the portlet URL containing this modified render state is activated,
the portlet will have the portlet mode and render parameter values set as they
were applied to the portlet URL.