Correct QP range when using ffmpeg VCE H265

By default ffmpeg restricts qp factors to a range of 2 to 31, unless otherwise overwritten. This significantly constrains the low bitrate performance of VCE in H.264 mode.
This should fix issue HandBrake#3447 and could also address parts of issue HandBrake#2980.

libhb/encavcodec.c

@@ -343,6 +343,15 @@ int encavcodecInit( hb_work_object_t * w, hb_job_t * job )
 
         if ( job->vcodec == HB_VCODEC_FFMPEG_VCE_H264 || job->vcodec == HB_VCODEC_FFMPEG_VCE_H265 ) {
             av_dict_set( &av_opts, "rc", "vbr_peak", 0 );
+            //Work around an ffmpeg issue mentioned in issue #3447
+            if (job->vcodec == HB_VCODEC_FFMPEG_VCE_H265)
+            {
+               av_dict_set( &av_opts, "min_qp_i", 1, 0 );
+               av_dict_set( &av_opts, "min_qp_p", 1, 0 );
+
+               av_dict_set( &av_opts, "max_qp_i", 51, 0 );
+               av_dict_set( &av_opts, "max_qp_p", 51, 0 );
+            }
             hb_log( "encavcodec: encoding at rc=vbr_peak Bitrate %d", job->vbitrate );
         }