for faster render settings tweaking, keep scene in VRAM after rendering until user goes back to 3d view

Keep scene in VRAM after rendering until user goes back to 3d view. This will be incredibly helpful when tweaking render settings, as the user won't have to wait for the scene to be re-calculated and put back onto the GPU.

Quite frustrating at the moment for large scenes when all you need to do is experiment with the render settings to find the right values to clear noise (particularly if you have a lot of scattered geometry).