avcodec: add littlebit options that are adjusted for VP8

Seems that maxrate needs somekinda exponential function from bitrate (comparing
ffmpeg's libvpx presets) and cbr isn't something you likely will get out of this.

modules/codec/avcodec/encoder.c

@@ -598,12 +598,18 @@ int OpenEncoder( vlc_object_t *p_this )
        libvpx-720p preset from libvpx ffmpeg-patch */
     if( i_codec_id == CODEC_ID_VP8 )
     {
-        p_context->bit_rate_tolerance = 2* p_enc->fmt_out.i_bitrate;
+        p_context->bit_rate_tolerance = __MAX(2 * p_enc->fmt_out.i_bitrate, p_sys->i_vtolerance );
+        /* I used Harrison-stetson method here to get there values */
+        p_context->rc_max_rate = 3 * p_enc->fmt_out.i_bitrate;
+        p_context->rc_min_rate = p_enc->fmt_out.i_bitrate / 200;
+
+
         p_context->lag = 16;
         p_context->level = 216;
         p_context->profile = 0;
         p_context->rc_buffer_aggressivity = 0.95;
         p_context->token_partitions = 4;
+        p_context->mb_static_threshold = 0;
     }
 #endif