Use the middle button instead left for camera navigation

[pbrt4bounty]

I have create a proof of concept in my fork to add menu options using Imgui so, I need to change the use of the middle mouse button in GUI mode to release the left button to other tasks, like, manage menus. With the current state, every time that you pick into a option menu, the render is reset. I have create a pull request here https://github.com/mmp/pbrt-v4/pull/448