Add more BRDFs

[n8xm]

It would be nice if we had some more BRDFs to play with. It should be easy to add more BRDFs in Disney's format - they just have to be modified slightly to conform to OpenGL ES rather than desktop OpenGL. The easiest way to start is to load a Disney .brdf file, see what breaks, then change it.

Disney's files are here: https://github.com/wdas/brdf/tree/master/src/brdfs .

Perhaps this task could be automated with a script? I believe that in the general case, converting desktop OpenGL to OpenGL ES is nontrivial. Also, the total number of files that Disney has is small, so it may be faster to just do it by hand than it is to write a foolproof script.