Everything still uses a single shader, but I've now implemented a much better glossy BRDF.

It's based on the "Generalized Trowbridge-Reitz" or GTR microfacet distribution presented in Disney's Principled BRDF paper. GTR is pretty much identical to GGX, except that it has an additional parameter to tweak the tail of the specular highlight.

There are still some bugs in the implementation (see, for example, the black spot on the lip of the teacup). And I'm still struggling to figure out certain things about microfacet BRDF's in general. But it's more-or-less working, with proper importance sampling and everything.

I'm especially pleased to see it working in the context of the curved surfaces, since they're rendered using micro-geometry. I'm not seeing any artifacts originating from that.