duke3d: further optimize audio mixing

Rather than holding intermediate results as fixed-point, this converts them
directly to normal integers (in the range of the PCM sample) while mixing,
instead of waiting till the end to perform a separate shifting step. Also,
this precalculates some constants in the reverb code.

Change-Id: Ie04e444d145bc28ce67eef9ae0ead6d328acf28a
5 files changed