lavfi/buffersrc: set min and max values for pix_fmt option

Currently the min and max values are not honored, but this will change in
a following patch.

libavfilter/buffersrc.c

@@ -326,7 +326,7 @@ static const AVOption buffer_options[] = {
     { "width",         NULL,                     OFFSET(w),                AV_OPT_TYPE_INT,      { .i64 = 0 }, 0, INT_MAX, V },
     { "video_size",    NULL,                     OFFSET(w),                AV_OPT_TYPE_IMAGE_SIZE,                .flags = V },
     { "height",        NULL,                     OFFSET(h),                AV_OPT_TYPE_INT,      { .i64 = 0 }, 0, INT_MAX, V },
-    { "pix_fmt",       NULL,                     OFFSET(pix_fmt),          AV_OPT_TYPE_PIXEL_FMT,                 .flags = V },
+    { "pix_fmt",       NULL,                     OFFSET(pix_fmt),          AV_OPT_TYPE_PIXEL_FMT, { .i64 = AV_PIX_FMT_NONE }, .min = AV_PIX_FMT_NONE, .max = INT_MAX, .flags = V },
 #if FF_API_OLD_FILTER_OPTS
     /* those 4 are for compatibility with the old option passing system where each filter
      * did its own parsing */