Fix loading Matroska index files on x64

OriginalPos is an unsigned integer of inconsistent size which can be
negative (due to being delta-coded), which makes the exact order of the
casts and manipulations very important.
Fix saving and loading audio indexes
Simplify the FPSDenominator calculation

Eliminates a rounding error in a specific 25 FPS sample file. Hopefully
does not introduce rounding errors for other samples.