resume image rendering in cycle

It could be very usefull to be able to resume image rendering in cycle like in last version of PRman:

renderman.pixar.com/resources/current/RenderMan/risRecovery.html

This is a really usefull function for previsualisation of entire animation with lighting and rendering. It become possible to work on compositing before the full quality rendering is calculated.

There could be an option in the render settings to save information to be able to resume the computation but this setting must be accesible by scripting for automatic previsualisation on renderfarm (night computation of entire movie in low quality for exemple) These previz-quality computation give information of rendering time for each frame that can be stored for estimate the full amount of time needed for the entire film.

the quality could be limited by number of samples or rendering time of frame.