lavfi/graphparser: improve applying graph-level sws options

The current code will apply them if the options string does not contain
a 'flags' substring, and will do so by appending the graph-level option
string to the filter option string (with the standard ':' separator).
This is flawed in at least the following ways:
- naive substring matching without actually parsing the options string
  may lead to false positives (e.g. flags are specified by shorthand)
  and false negatives (e.g. the 'flags' substring is not actually the
  option name)
- graph-level sws options are not limited to flags, but may set
  arbitrary sws options

This commit simply applies the graph-level options with
av_set_options_string() and lets them be overridden as desired by the
user-specified filter options (if any). This is also shorter and avoids
extra string handling.

libavfilter/graphparser.c

@@ -25,6 +25,8 @@
 
 #include "libavutil/avstring.h"
 #include "libavutil/mem.h"
+#include "libavutil/opt.h"
+
 #include "avfilter.h"
 
 #define WHITESPACES " \n\t\r"
@@ -101,7 +103,6 @@ static int create_filter(AVFilterContext **filt_ctx, AVFilterGraph *ctx, int ind
     const AVFilter *filt;
     char name2[30];
     const char *inst_name = NULL, *filt_name = NULL;
-    char *tmp_args = NULL;
     int ret, k;
 
     av_strlcpy(name2, name, sizeof(name2));
@@ -136,16 +137,10 @@ static int create_filter(AVFilterContext **filt_ctx, AVFilterGraph *ctx, int ind
         return AVERROR(ENOMEM);
     }
 
-    if (!strcmp(filt_name, "scale") && (!args || !strstr(args, "flags")) &&
-        ctx->scale_sws_opts) {
-        if (args) {
-            tmp_args = av_asprintf("%s:%s",
-                    args, ctx->scale_sws_opts);
-            if (!tmp_args)
-                return AVERROR(ENOMEM);
-            args = tmp_args;
-        } else
-            args = ctx->scale_sws_opts;
+    if (!strcmp(filt_name, "scale") && ctx->scale_sws_opts) {
+        ret = av_set_options_string(*filt_ctx, ctx->scale_sws_opts, "=", ":");
+        if (ret < 0)
+            return ret;
     }
 
     ret = avfilter_init_str(*filt_ctx, args);
@@ -159,7 +154,6 @@ static int create_filter(AVFilterContext **filt_ctx, AVFilterGraph *ctx, int ind
         *filt_ctx = NULL;
     }
 
-    av_free(tmp_args);
     return ret;
 }