Implement sse::clamp() with fewer instructions
Use min instead of clamp at volume borders
Less indirection in inner ray-casting loop

- Avoid expensive std::vector::operator[], which is not
  inlined by icpc
- Declare variables more globally, already having Xeon Phi
  offloading in mind
Replace task queue with atomic counters and cond's

- no CPU power used while threads idle due to signal and wait
- atomic counter much more lightweight - that's actually not
  an issue with host CPUs, but is mandatory on the Xeon Phi