Cosa è cambiato in EDuke32, build per build

Currently, the FPS determination is based on libvpx's vpxdec.c code, which uses the FPS provided in the IVF file in one case, and simply sets it to 30 FPS in the other. For the first case, a "correction" is carried out for something which the comments suggest to originate from other (old?) VPX encoder versions.

three individual loops and compile the enclosing function at -O3 (-O1 for debugging builds). Now, the time for this conversion ranges from 7 to 18 ms per frame across various tested machines, a clear improvement.