Mesh size limits

[kpreid]

fuzz_mesh is hitting the fuzzer-imposed memory limit. We should place explicit limits on the maximum complexity of a mesh so that it not only doesn't hit the fuzzer limit, but is also reasonable for actual rendering.

[kpreid]

The fuzzer out-of-memory turned out to be not from mesh generation, but from generating a huge LightStorage::propagation_table. I fixed that by constraining the value in 217bd70. However, mesh size limits are still desirable.