Speed up ffmpeg format probing if we'll trust codec fps for playback

This small change allows you to (re)use the pre-exiting advancedsettings.xml 'fpsdetect' option in order to reduce the playback starting time. 

Until now, 'fpsdetect' was only used in the video players, with values:

0 = trust codec fps
1 = recalculate from video timestamps with uniform spacing
2 = recalculate from video timestamps always

With this patch AVFormatContext field 'fps_probe_size' will be 0 when we trust codec fps, avoiding spend up to 40 frames¹ per video stream recalculating the framerate. 

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¹: https://github.com/xbmc/xbmc/blob/master/lib/ffmpeg/libavformat/utils.c#L2606