fix mpeg4 video encoding on win32

some devices are providing their framerate ratios in a format
incompatible with FFmpeg encoder.
This patch sanitizes these entries to fit the FFmpeg code.

Change-Id: Ib25c33699749ddcdfdf9cefc853f24caae433609
Reviewed-by: Guillaume Roguez <guillaume.roguez@savoirfairelinux.com>