Add support for non-square render resolutions (width != height)

[lasagnaphil]

This is a working draft of a PR that adds support for rasterizing to images of non-square size (width != height).

I've modified the CUDA kernels and C++ functions, changed the Rasterizer class to accept (width, height) tuple for image_size, and have confirmed that demo_render.py is working with other image sizes. I've yet not tested the other demos though, and there might be other places in the Python API that I've missed that needs to be changed as well.

This effectively solves issue #34.

The things left to do:

• Add support for non-square image sizes in C++/CUDA land
• Rasterizer class accepts (width, height) tuple for image_size parameter
• Modify other places in the Python API (such as the Camera class) to also work with non-square resolution
• Check if all Python demos are working as expected (even with different image resolutions)

Since I have limited spare time, any help on completing this PR can be appreciated (especially modifying and testing it on the Python API side).