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Fix T68343: Rendered video plays at 600fps

Field time_base of video stream must be set for some containers,
otherwise avformat_write_header() will set it to default values.
Rendered file in such case won't be played at desired frame rate.

See init_muxer() in mux.c in ffpmeg sources.

Reviewed By: sergey

Differential Revision: https://developer.blender.org/D9213