guard all window references

[lindapaiste]

In order to run in Node.js we must guard all window references with typeof window !== 'undefined' before accessing the window variable. Otherwise it throws a fatal error ReferenceError: window is not defined as reported in issue #1317.

This PR addresses all uses of window in the /src directory:

• Copies some of the code from my pending PR Code clean-up: class-methods-use-this #1312 which guards window.location.pathname in the modelLoader utility and removes it from the FaceApi class.
• Accesses Color class through p5Utils.p5Instance.Color instead window.p5.Color in BodyPix. (Note: this was previously guarded by p5Utils.checkP5, so it would only cause an error if the user was using p5 in Node).
• Adds a guard around window in the P5Util constructor. There was already a guard in the P5Util.p5Instance getter, but I believe that is too late as the error will have already been thrown in the constructor.

There will be additional changes needed to run in Node.js, such as guarding classes HTMLImageElement, etc.

[joeyklee]

Having reviewed #1319 -- I'm going to give this one a 👍 !! Thanks so much for these incremental PRs ❤️