Fix CameraManager.GetCamera() hanging and FrameConverter crashing

[Speykious]

Maybe it would have been more organized to separate these in 2 different PRs, but the changes made are so small I just decided to do it that way.

This PR fixe #9.
It turns out that I don't even need to call ffmpeg.avformat_find_stream_info(). From the documentation:

Read packets of a media file to get stream information. This is useful for file formats with no headers such as MPEG. This function also computes the real framerate in case of MPEG-2 repeat frame mode. The logical file position is not changed by this function; examined packets may be buffered for later processing.
It looks like it just works now. The camera gets retrieved but the Width and Height are at 0.
I wonder if I should make a warning, or have some way of detecting if the camera can get decoded.

This PR also fixes #10.
As for the FrameConverter, all I did was throwing an ArgumentException if any of the Width/Height properties are at 0.

[adryzz]

LGTM