lavfi/spp: add "quality" option in runtime change path

it's stranage to use option "level" in runtime change path but used
"quality" in option, add "quality" in runtime change path, it's more
intuitive and keep the "level" for compatibility.
Reviewe-by: Michael Niedermayer <michael@niedermayer.cc>
Signed-off-by: Jun Zhao <barryjzhao@tencent.com>

doc/filters.texi

@@ -17290,6 +17290,15 @@ option may cause flicker since the B-Frames have often larger QP. Default is
 @code{0} (not enabled).
 @end table
 
+@subsection Commands
+
+This filter supports the following commands:
+@table @option
+@item level
+@item quality
+Same as quality option. And the command accepts the @code{max} same as the @code{6}.
+@end table
+
 @section sr
 
 Scale the input by applying one of the super-resolution methods based on

libavfilter/vf_spp.c

@@ -444,7 +444,7 @@ static int process_command(AVFilterContext *ctx, const char *cmd, const char *ar
 {
     SPPContext *s = ctx->priv;
 
-    if (!strcmp(cmd, "level")) {
+    if (!strcmp(cmd, "level") || !strcmp(cmd, "quality")) {
         if (!strcmp(args, "max"))
             s->log2_count = MAX_LEVEL;
         else