now with 50% less computation!

* avoiding modulo in frame_index
* using compile time BYTES_PER_PIXEL takes advantage of compiler loop unrolling or copy optimizations or something. it gets an extra 5% or so.
* avoiding ofMap

new code takes 50% as long without blending, 57% with blending. for reference, this means around 24ms to do 720p with blending on my laptop.  

this is really the perfect place for openmp or tbb :(