All current renderers and muxers have no issues with longer timestamps.
However, classic VSFilter keeping track of time with a 32-bit int,
leads to a more meaningful limit at 596h, so enforce that instead.
Since the stringification code for SRT and ASS (but not SMPTE) relied on
hours being single digit, replace them with a simple format string.
GUI inputs are still limited to <10h, but at least
pre-existing timestamps no longer get mangled.