Renderwindow viewport fade?

I suggest adding a slider feature to Blender that allows users to fade in the current viewport image while or after rendering. This would function similarly to the slot system but with a smooth slider to blend between the render and the viewport.

It would help speed up render debugging and iteration by making it easier to compare the rendered image with the viewport while rendering.

This idea is inspired by SGI workstations, and since most Blender scenes aren't rendered in small tiles anymore, a simple fade slider would be more effective than rendering over viewport frames.

This would need an option to turn it on or off so that no one can be mad that it eats vram :)