libx264: fix setting some options.

Specifically: gop_size, max_b_frames, scenechange_threshold, qmin, qmax,
max_qdiff, qblur, qcompress and refs.

Change their default values to -1 and only use them if the user
explicitly set them. Otherwise x264 defaults are used.

Move setting those options after x264_param_default_preset(), so they
don't get overwritten by it.