Updated printScreen method to lookup bpp value.

The bpp displaymode query returns 0 in windowed mode, or actuall bpp
in fullscreen mode. If the 0 value is detected in non-fullscreen mode, the
logic queries the display for the DesktopDisplayMode.  Finally if value is
still at 0 bpp, overrides with 32 (equivalent to original 4 value that was
hardcoded for RGBA).  Updated values relying on the bpp value to take into
account actual byte count value from bits by dividing by 8.
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.
Merge pull request #519 from emenifee/develop

Screenshot optimization merge