sub.ImageFile: Fix the end time of some PGS subtitles
sub.ImageFile: Fix the end time of some PGS subtitles

Any subtitle that's supposed to be displayed right up until the next one
did not have the correct end time.

Further fixes the rendering of a file from #299.

vapoursynth/vapoursynth,Kamekameha/vapoursynth,vapoursynth/vapoursynth,Kamekameha/vapoursynth,Kamekameha/vapoursynth,vapoursynth/vapoursynth,Kamekameha/vapoursynth,vapoursynth/vapoursynth