Fix: Export subtitles timecode relative to scene start frame, ignore muted strips

This patch writes the timecode in the .srt file relative to the start
frame of the scene. If the timecode is global but scene does not start
at frame 0 the subtitles don't match if they get loaded in an external
video player. Muted strips will be ignored. Don't allow negative
timecodes in .srt.

Reviewed By: Richard Antalik
Differential Revision: https://developer.blender.org/D11762