Make separate MIS environment settings for Render and Viewport

When using HDRI lighting, it is necessary to use high values for the Map Resolution in the Multiple Importance section of the World panel to get good results. These high values cause a delay before rendering begins ("Updating Lights | Importance Map"), which is fine when performing final or test Renders, but is very annoying when using Rendered Mode previews in the viewport. This results in the user having to manually lower the value while working on the scene, increase it again to do a test render, turn it back down to make further edits, and so on. It would be very helpful to have the option to use different MIS Map Resolution values for Render and Viewport views (similar to the Subsurf modifier), so the user doesn't have to manually change it back and forth repeatedly.