support setting image_size=(width, height)

[hzsydy]

No description provided.

[nkolot]

Did you verify that it still passes the existing test cases? I cannot test it myself for the next few days.

[hzsydy]

Even daniilidis-group/neural_renderer branch itself cannot pass the test cases.
Renderer.render() now returns a tuple instead of a single image, so most of the test cases just failed.

[hzsydy]

fix a bug in orig_size I wrote, which is not tested in testcase lol

[nkolot]

Sorry for not being very responsive. What is the status of this PR?

[hzsydy]

I believe it works (because I am using it now), but not exactly sure.

[nkolot]

From a quick look it doesn't not break backwards compatibility. I will try to write some small unit tests, and if it passes I will merge it.

[EricCousineau-TRI]

FWIW I think I've done some basic fixing of unittests in #115 (well, relaxing the failing mask intersections at least)