Fixed screen twitches with "shakes" sound shader

Caused by inaccuracies in the precached amplitude data in the OpenAL
backend, see https://github.com/dhewm/dhewm3/pull/71 for more details.

Video sample (not mine):
http://www.youtube.com/watch?v=ZUohifAbPW0
The "twitches" can also be observed right at the beginning of the
mars_city2 map.