avcodec/libx264: fix chroma quantizer offset usage

The default for the chromaoffset field in AVCodecContext
is zero, which until now always ended up overriding the
AVOption-set value, thus leading to the AVOption not working.

Additionally, the previous usage prevented the usage of
negative values, while both the variable as well as x264's
API would successfully handle such.

Thus, the default value of the AVOption is changed to match
the default of x264 (and what is currently the default for
the AVCodecContext chromaoffset field), and the checks are
changed to check for nonzero values.

This way:
1. the library default is still utilized if the value is zero.
2. both negative and positive values are correctly passed to
   x264.

For historical context, this was initially similarly
implemented in 5764d38173, and
then b340bd8a58 broke the
value.

Partially reverts commit b340bd8a58.

Signed-off-by: Takio Yamaoka <y.takio@gmail.com>

libavcodec/libx264.c

@@ -681,11 +681,11 @@ static av_cold int X264_init(AVCodecContext *avctx)
 
 #if FF_API_PRIVATE_OPT
 FF_DISABLE_DEPRECATION_WARNINGS
-    if (avctx->chromaoffset >= 0)
+    if (avctx->chromaoffset)
         x4->chroma_offset = avctx->chromaoffset;
 FF_ENABLE_DEPRECATION_WARNINGS
 #endif
-    if (x4->chroma_offset >= 0)
+    if (x4->chroma_offset)
         x4->params.analyse.i_chroma_qp_offset = x4->chroma_offset;
 
     if (avctx->gop_size >= 0)
@@ -1140,7 +1140,7 @@ static const AVOption options[] = {
     { "vlc",              NULL, 0, AV_OPT_TYPE_CONST, { .i64 = 0 },  INT_MIN, INT_MAX, VE, "coder" },
     { "ac",               NULL, 0, AV_OPT_TYPE_CONST, { .i64 = 1 },  INT_MIN, INT_MAX, VE, "coder" },
     { "b_strategy",   "Strategy to choose between I/P/B-frames",          OFFSET(b_frame_strategy), AV_OPT_TYPE_INT, { .i64 = -1 }, -1, 2, VE },
-    { "chromaoffset", "QP difference between chroma and luma",            OFFSET(chroma_offset), AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX, VE },
+    { "chromaoffset", "QP difference between chroma and luma",            OFFSET(chroma_offset), AV_OPT_TYPE_INT, { .i64 = 0 }, INT_MIN, INT_MAX, VE },
     { "sc_threshold", "Scene change threshold",                           OFFSET(scenechange_threshold), AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX, VE },
     { "noise_reduction", "Noise reduction",                               OFFSET(noise_reduction), AV_OPT_TYPE_INT, { .i64 = -1 }, INT_MIN, INT_MAX, VE },