Fix 3DO texture distortion

[MHeasell]

We triangulate 3do meshes before display. This causes texture distortion on quads that are trapezium shaped.

Some options:

1. render GL_QUADS (deprecated) and hope they interpolate as expected
2. subdivide the mesh
3. consider if we can do some maths to compute the texture coordinate correctly

[MHeasell]

Maybe wait for #4 to give us more flexibility.

[MHeasell]

4 has made enough progress that it no longer blocks this. We can do this in shaders with appropriate inputs from the outside.

Some useful reading:

http://web.cs.ucdavis.edu/~amenta/s12/perspectiveCorrect.pdf http://www.reedbeta.com/blog/quadrilateral-interpolation-part-1/ http://www.reedbeta.com/blog/quadrilateral-interpolation-part-2/ https://stackoverflow.com/questions/14971282/applying-color-to-single-vertices-in-a-quad-in-opengl

In particular, the third and fourth links describe implementing bilinear interpolation across a quad (not projective interpolation), I think something like this would be most appropriate here.

[MHeasell]

I should note that implementation of the above is a bit more involved than I had hoped so this will likely be de-prioritised until the engine has more features.

[MHeasell]

The following article improves upon Nathan Reed's solution for bilinear interpolation by also considering perspective correction:

http://www.unchainedgeometry.com/blog-1.html