optimize_png: Reduce OptiPNG optimization level

According to docs on the OptiPNG homepage "30-40 trials should be
satisfactory to all users, for it is very, very unlikely to be beaten
significantly by any wider search. The rest of the trial configurations
... were used in the experimentation from which we concluded they are
indeed useless!"

Go from 240 trails to 48, to speed up the optimzation process.
Testing on 703 images, shows the reduction in trials shows no
difference in size between o7 and o5.

Signed-off-by: Luis Cruz <ljc2491@gmail.com>
optimize_png: Rename for easier usagae

Signed-off-by: Luis Cruz <ljc2491@gmail.com>