Removed an unused cntr variable, and marked down a BUG for later fixing
Optimizations:
* Call filter function once per buffer read, not once per sample. Makes loops run tighter.
* Reduce pointer dereferencing, use array access instead.
* Removed a couple of unused variables.
* Documented two additional places of potential optimisation.
About 50% speedup on Raspberry Pi (CPU use 75% => 50%).
receiver: small optimisation cleanup