p5.Camera Reorganization

[AidanNelson]

Rebased p5.Camera Reorganization from #3063.

Updates:

• several documentation updates,
• adds an _isActive() helper method to p5.Camera prototype, called before each update to the renderer in ortho(), perspective(), or camera() to support multiple cameras,
• fixes resize issue: current behavior has default cameras updating both eyeZ and aspectRatio during resize, whereas custom cameras only update aspectRatio,
• removes return this; from constructor.

[kjhollen]

Hey @AidanNelson this looks ready! Nice work with the rebase. There are some conflicts with the webgl-gsoc-2018 branch because it looks like some edits were made to camera.js since your starting point and also to p5.RendererGL.js. Do you want to have a go at fixing those? You can try to merge with webgl-gsoc-2018 locally and then pushing the code again.

I'll merge this as-is once the conflicts are resolved, but it also feels like time to write some unit tests for p5.Camera as the next big step!

[kjhollen]

🎉 unit testing time!