avidmxfinfo: partial fix for start timecode for 29.97hz sources
avidmxfinfo: partial fix for start timecode for 29.97hz sources

Removed incorrect conversion using rounded timecode base and edit rate and
instead identify and support 2 most likely cases

stuarthicks/libmxf,Limecraft/ebu-libmxf,ebu/ebu-libmxf,ebu/ebu-libmxf,Limecraft/ebu-libmxf,Limecraft/ebu-libmxf,ebu/ebu-libmxf,stuarthicks/libmxf,stuarthicks/libmxf,ebu/ebu-libmxf,stuarthicks/libmxf