avfilter/graphparser: allow specifying filter@id as filter instance

See http://lists.ffmpeg.org/pipermail/ffmpeg-user/2017-April/035975.html
Parsed_filter_X could remain and user can override it with custom one.

Example:
ffplay -f lavfi "nullsrc=s=640x360,
sendcmd='1 drawtext@top reinit text=Hello; 2 drawtext@bottom reinit text=World',
drawtext@top=x=16:y=16:fontsize=20:fontcolor=Red:text='',
drawtext@bottom=x=16:y=340:fontsize=16:fontcolor=Blue:text=''"
Reviewed-by: Paul B Mahol <onemda@gmail.com>
Signed-off-by: Muhammad Faiz <mfcc64@gmail.com>

doc/filters.texi

@@ -130,11 +130,11 @@ filterchains is represented by a list of ";"-separated filterchain
 descriptions.
 
 A filter is represented by a string of the form:
-[@var{in_link_1}]...[@var{in_link_N}]@var{filter_name}=@var{arguments}[@var{out_link_1}]...[@var{out_link_M}]
+[@var{in_link_1}]...[@var{in_link_N}]@var{filter_name}@@@var{id}=@var{arguments}[@var{out_link_1}]...[@var{out_link_M}]
 
 @var{filter_name} is the name of the filter class of which the
 described filter is an instance of, and has to be the name of one of
-the filter classes registered in the program.
+the filter classes registered in the program optionally followed by "@@@var{id}".
 The name of the filter class is optionally followed by a string
 "=@var{arguments}".
 
@@ -212,10 +212,11 @@ to the filtergraph description.
 Here is a BNF description of the filtergraph syntax:
 @example
 @var{NAME}             ::= sequence of alphanumeric characters and '_'
+@var{FILTER_NAME}      ::= @var{NAME}["@@"@var{NAME}]
 @var{LINKLABEL}        ::= "[" @var{NAME} "]"
 @var{LINKLABELS}       ::= @var{LINKLABEL} [@var{LINKLABELS}]
 @var{FILTER_ARGUMENTS} ::= sequence of chars (possibly quoted)
-@var{FILTER}           ::= [@var{LINKLABELS}] @var{NAME} ["=" @var{FILTER_ARGUMENTS}] [@var{LINKLABELS}]
+@var{FILTER}           ::= [@var{LINKLABELS}] @var{FILTER_NAME} ["=" @var{FILTER_ARGUMENTS}] [@var{LINKLABELS}]
 @var{FILTERCHAIN}      ::= @var{FILTER} [,@var{FILTERCHAIN}]
 @var{FILTERGRAPH}      ::= [sws_flags=@var{flags};] @var{FILTERCHAIN} [;@var{FILTERGRAPH}]
 @end example
@@ -17246,6 +17247,12 @@ Specify audio tempo change at second 4:
 asendcmd=c='4.0 atempo tempo 1.5',atempo
 @end example
 
+@item
+Target a specific filter instance:
+@example
+asendcmd=c='4.0 atempo@@my tempo 1.5',atempo@@my
+@end example
+
 @item
 Specify a list of drawtext and hue commands in a file.
 @example

libavfilter/graphparser.c

@@ -88,20 +88,36 @@ static char *parse_link_name(const char **buf, void *log_ctx)
  * @param filt_ctx put here a filter context in case of successful creation and configuration, NULL otherwise.
  * @param ctx the filtergraph context
  * @param index an index which is supposed to be unique for each filter instance added to the filtergraph
- * @param filt_name the name of the filter to create
+ * @param name the name of the filter to create, can be filter name or filter_name\@id as instance name
  * @param args the arguments provided to the filter during its initialization
  * @param log_ctx the log context to use
  * @return >= 0 in case of success, a negative AVERROR code otherwise
  */
 static int create_filter(AVFilterContext **filt_ctx, AVFilterGraph *ctx, int index,
-                         const char *filt_name, const char *args, void *log_ctx)
+                         const char *name, const char *args, void *log_ctx)
 {
     AVFilter *filt;
-    char inst_name[30];
+    char name2[30];
+    const char *inst_name = NULL, *filt_name = NULL;
     char *tmp_args = NULL;
-    int ret;
+    int ret, k;
+
+    av_strlcpy(name2, name, sizeof(name2));
+
+    for (k = 0; name2[k]; k++) {
+        if (name2[k] == '@' && name[k+1]) {
+            name2[k] = 0;
+            inst_name = name;
+            filt_name = name2;
+            break;
+        }
+    }
 
-    snprintf(inst_name, sizeof(inst_name), "Parsed_%s_%d", filt_name, index);
+    if (!inst_name) {
+        snprintf(name2, sizeof(name2), "Parsed_%s_%d", name, index);
+        inst_name = name2;
+        filt_name = name;
+    }
 
     filt = avfilter_get_by_name(filt_name);