libx264: change i_qfactor to use x264cli's default

This also allows libx264 to modify its i_qfactor value
when using the "-tune" setting. Previously it had a static
value of 1.25

Signed-off-by: Anton Khirnov <anton@khirnov.net>

libavcodec/libx264.c

@@ -279,7 +279,8 @@ static av_cold int X264_init(AVCodecContext *avctx)
             (float)avctx->rc_initial_buffer_occupancy / avctx->rc_buffer_size;
     }
 
-    x4->params.rc.f_ip_factor             = 1 / fabs(avctx->i_quant_factor);
+    if (avctx->i_quant_factor > 0)
+        x4->params.rc.f_ip_factor         = 1 / fabs(avctx->i_quant_factor);
     x4->params.rc.f_pb_factor             = avctx->b_quant_factor;
     x4->params.analyse.i_chroma_qp_offset = avctx->chromaoffset;
 
@@ -559,6 +560,7 @@ static const AVCodecDefault x264_defaults[] = {
     { "b",                "0" },
     { "bf",               "-1" },
     { "g",                "-1" },
+    { "i_qfactor",        "-1" },
     { "qmin",             "-1" },
     { "qmax",             "-1" },
     { "qdiff",            "-1" },