libx264: Set the default of the rc_lookahead option to -1

This allows it to use the defaults specified by preset/tune,
without overwriting it with the default value from the
AVCodecContext field.
Signed-off-by: Martin Storsjö <martin@martin.st>

libavcodec/options.c

@@ -492,7 +492,7 @@ static const AVOption options[]={
 {"psy_trellis", "specify psycho visual trellis", OFFSET(psy_trellis), AV_OPT_TYPE_FLOAT, {.dbl = -1 }, -1, FLT_MAX, V|E},
 {"aq_mode", "specify aq method", OFFSET(aq_mode), AV_OPT_TYPE_INT, {.dbl = -1 }, -1, INT_MAX, V|E},
 {"aq_strength", "specify aq strength", OFFSET(aq_strength), AV_OPT_TYPE_FLOAT, {.dbl = -1.0 }, -1, FLT_MAX, V|E},
-{"rc_lookahead", "specify number of frames to look ahead for frametype", OFFSET(rc_lookahead), AV_OPT_TYPE_INT, {.dbl = 40 }, 0, INT_MAX, V|E},
+{"rc_lookahead", "specify number of frames to look ahead for frametype", OFFSET(rc_lookahead), AV_OPT_TYPE_INT, {.dbl = -1 }, -1, INT_MAX, V|E},
 {"ssim", "ssim will be calculated during encoding", 0, AV_OPT_TYPE_CONST, {.dbl = CODEC_FLAG2_SSIM }, INT_MIN, INT_MAX, V|E, "flags2"},
 {"intra_refresh", "use periodic insertion of intra blocks instead of keyframes", 0, AV_OPT_TYPE_CONST, {.dbl = CODEC_FLAG2_INTRA_REFRESH }, INT_MIN, INT_MAX, V|E, "flags2"},
 {"crf_max", "in crf mode, prevents vbv from lowering quality beyond this point", OFFSET(crf_max), AV_OPT_TYPE_FLOAT, {.dbl = DEFAULT }, 0, 51, V|E},