Fix wrong FFmpeg chapter export TIMEBASE

See bug 487019:

When exporting guides to FFmpeg chapters, the TIMEBASE option is swapped. I get 25/1 instead of 1/25.

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a bit out of context:

Another thing I observed while testing is that if there is no clip in the timeline, the END of the last marker/chapter will be -1 and ffmpeg will throw with

[ffmetadata @ 0x5b8d607efec0] Chapter end time -1 before start 930
[in#1 @ 0x5b8d605ec000] Error opening input: Cannot allocate memory
Error opening input file test_guides.txt.
Error opening input files: Cannot allocate memory

If the last marker is after the last clip end, END will be less than START, I haven’t tested what this does to ffmpeg.

Though probably nobody will create markers after the last clip end I don’t think this is too relevant (and I wouldn’t know how to fix it).