lavc: make rc_qsquish a private option of mpegvideo encoders
lavc: make rc_qmod_* into private options of mpegvideo encoders
lavc: make rc_eq into private options of mpegvideo encoders
lavc: make rc_buffer_aggressivity/rc_initial_cplx into private options of mpegvideo encoders
lavc: deprecate unused me_threshold field
libvpxenc: add static-thresh private option

Currently, this option is accessed through AVCodecContext.mb_threshold,
which originally controlled reusing MB data when transcoding mpeg to
mpeg. Since the libvpx meaning is completely different from the original
mpegvideo meaning, it is better to use a separate private option for
this.
lavc: deprecate unused mb_threshold field
lavc: make border_masking into private options of mpegvideo encoders
lavc: make lmax/lmin into private options of mpegvideo encoders