Renamed variables to make since with pre-calculated bytes per pixel.

Reduced math calculation in worker loop/buffer allocation.  Originally
bpp was treated as bytes per pixel instead of bits per pixel.  When I
fixed the lookup for obtaining bpp I altered uses to calculate bytes
inline from bits->byte with /8.  This left the extra constant division
in the worker loop.  Since everything else that cared about bpp really
only cared about number of bytes; I renamed and pre-calculated this value.