Update nanogui and use our own mainloop

[petterarvidsson]

To make it easy to follow nanogui development, remove our changes and instead implement a simple main loop of our won based on: https://github.com/wjakob/nanogui/blob/master/src/example3.cpp

Also replace nanogui dependency with the one directly from https://github.com/wjakob/nanogui

[petterarvidsson]

@nsg What do you think?

[petterarvidsson]

Apparently they moved from glew to glad for windows compatibility... ugh...

[nsg]

@petterarvidsson Looks good, a little work to keep all these callbacks up-2-date in the future but over all this is a good thing.