move param fixups to Encoder::configure()
update/fix comments
white-space fixes, reorder for clarity

--HG--
extra : amend_source : c06f2b87987931998956976d7b7c451b5268f4f3
common: remove trailing white-space
common: do not report ssim costs by default

It costs CPU cycles to measure SSIM, do not do this unless the user asks for
them with --ssim