Make relayout work for scene.camera

[etpinard]

fixes #934

so that http://codepen.io/monfera/pen/KMmPPV can now use Plotly.relayout as

function rotate(id, angle) {
  var scene = gd._fullLayout[id];
  var camera = scene.camera;

  var rtz = xyz2rtz(camera.eye);
  rtz.t += angle;
  camera.eye = rtz2xyz(rtz);
  
  Plotly.relayout(gd, id + '.camera', camera);
}

instead of digging into the scene object internals.

---

In addition,

• by making relayout work for scene.camera we can now rewrite the 3d camera modebar handlers in a much cleaner way 604f7a0
• to ensure that camera relayout are as fast as they can be, a new relayout path docamera is added in 99ad3cb

[jackparmer]

ping @thejackluo , who was working a on remake of this NYT classic:
https://www.nytimes.com/interactive/2015/03/19/upshot/3d-yield-curve-economic-growth.html

[alexcjohnson]

why is this commit in a PR about 3D?

[etpinard]

5f36071 broke the test w/o me noticing. As test cases that require a gl context are skipped on CI, this went unnoticed until now.

[etpinard]

Full disclosure, I actually noticed this broken test case twice before (when releasing 1.22.0 and 1.23.0), but I was too lazy and didn't feel like making a PR just for that.

[alexcjohnson]

Very nice, much cleaner. 💃