wrong transcoder reset() use (ffmpeg backend)

I the current version, at least using the ffmpeg backend, when a transcode run fails the phpvideotoolkit reset() method is invoked. This way the input filename is resetted too, so also all the next transcode runs for the same video file fail.
My patch fix this issue simply enabling the keep_input_file option in the reset() method.

Another issue consists in missing reset() before each transcode, because of the actual TranscoderAbstractionFactoryFfmpeg:setOptions() implementation when an option in the preset video setting is equal to none or empty it is simply skipped... but if it has been set by a previous run it will be still wrongly set.

In my case the webm preset wrongly inherits the -vpre option from the mp4 preset stuff.
My patch in this case resets the transcoder class, keeping the input file info, before executing the transcode.