Fix LTC decoder timecode conversion

Dear Joker developers,

This pull request is about a bug observed with the LTC sync. We observed a drift when syncing with a LTC time source on a 24 fps video. The drift has been tracked down to the timecode-to-frame conversion in the LTC reader where the timecode type is hardcoded although the clock timecode type may change elsewhere in Joker.

Thanks for your attention !