Rework uniform setting.

One would think something as mundane as setting a few uniforms wouldn't
really mean much for performance, but seemingly this is not always so --
I had a real-world shader that counted no less than 55 uniforms.
Of course, not all of these were actually used, but we still have to go
through looking up the name etc. for every single one, every single frame.

Thus, we introduce a new way of dealing with uniforms: Register them before
finalization time, and then EffectChain can store their numbers once and
for all, instead of this repeated lookup. The system is also set up such
that we can go to uniform buffer objects (UBOs) in the very near future.

It's a bit unfortunate that uniform declaration now is removed from the
.frag files, where it sat very nicely, but the alternative would be to
try to parse GLSL, which I'm a bit wary at right now. All effects are
converted, leaving the set_uniform_* functions without any users, but
they are kept around for now in case external effects want them.

This seems to bring 1–2% speedup for my use case; hopefully UBOs will
bring a tiny bit more.