fftools/ffmpeg: _-prefix variables in MATCH_PER_STREAM_OPT()

Avoids conflicts, e.g. when 'i' is used in the outvar expression.

fftools/ffmpeg.h

@@ -783,26 +783,26 @@ InputStream *ist_iter(InputStream *prev);
 {\
     char namestr[128] = "";\
     const char *spec = so->specifier && so->specifier[0] ? so->specifier : "";\
-    for (i = 0; opt_name_##name[i]; i++)\
-        av_strlcatf(namestr, sizeof(namestr), "-%s%s", opt_name_##name[i], opt_name_##name[i+1] ? (opt_name_##name[i+2] ? ", " : " or ") : "");\
+    for (int _i = 0; opt_name_##name[_i]; _i++)\
+        av_strlcatf(namestr, sizeof(namestr), "-%s%s", opt_name_##name[_i], opt_name_##name[_i+1] ? (opt_name_##name[_i+2] ? ", " : " or ") : "");\
     av_log(NULL, AV_LOG_WARNING, "Multiple %s options specified for stream %d, only the last option '-%s%s%s "SPECIFIER_OPT_FMT_##type"' will be used.\n",\
            namestr, st->index, opt_name_##name[0], spec[0] ? ":" : "", spec, so->u.type);\
 }
 
 #define MATCH_PER_STREAM_OPT(name, type, outvar, fmtctx, st)\
 {\
-    int i, ret, matches = 0;\
+    int _ret, _matches = 0;\
     SpecifierOpt *so;\
-    for (i = 0; i < o->nb_ ## name; i++) {\
-        char *spec = o->name[i].specifier;\
-        if ((ret = check_stream_specifier(fmtctx, st, spec)) > 0) {\
-            outvar = o->name[i].u.type;\
-            so = &o->name[i];\
-            matches++;\
-        } else if (ret < 0)\
+    for (int _i = 0; _i < o->nb_ ## name; _i++) {\
+        char *spec = o->name[_i].specifier;\
+        if ((_ret = check_stream_specifier(fmtctx, st, spec)) > 0) {\
+            outvar = o->name[_i].u.type;\
+            so = &o->name[_i];\
+            _matches++;\
+        } else if (_ret < 0)\
             exit_program(1);\
     }\
-    if (matches > 1)\
+    if (_matches > 1)\
        WARN_MULTIPLE_OPT_USAGE(name, type, so, st);\
 }