common: change mp_snprintf_append semantics

Make it more suitable for chaining. This means a function formatting a
value to a string using a static buffer can work exactly like
mp_snprintf_append itself.

Also rename it to mp_snprintf_cat, because that's shorter.

common/common.c

@@ -109,16 +109,23 @@ bool mp_rect_intersection(struct mp_rect *rc, const struct mp_rect *rc2)
 
 // This works like snprintf(), except that it starts writing the first output
 // character to str[strlen(str)]. This returns the number of characters the
-// string would have assuming a large enough buffer, will make sure str is
-// null-terminated, and will never write to str[size] or past.
-int mp_snprintf_append(char *str, size_t size, const char *format, ...)
+// string would have *appended* assuming a large enough buffer, will make sure
+// str is null-terminated, and will never write to str[size] or past.
+// Example:
+//  int example(char *buf, size_t buf_size, double num, char *str) {
+//      int n = 0;
+//      n += mp_snprintf_cat(buf, size, "%f", num);
+//      n += mp_snprintf_cat(buf, size, "%s", str);
+//      return n; }
+// Note how this can be chained with functions similar in style.
+int mp_snprintf_cat(char *str, size_t size, const char *format, ...)
 {
     size_t len = strnlen(str, size);
     assert(!size || len < size); // str with no 0-termination is not allowed
     int r;
     va_list ap;
     va_start(ap, format);
-    r = len + vsnprintf(str + len, size - len, format, ap);
+    r = vsnprintf(str + len, size - len, format, ap);
     va_end(ap);
     return r;
 }

common/common.h

@@ -77,7 +77,7 @@ struct mp_rect {
 void mp_rect_union(struct mp_rect *rc, const struct mp_rect *src);
 bool mp_rect_intersection(struct mp_rect *rc, const struct mp_rect *rc2);
 
-int mp_snprintf_append(char *str, size_t size, const char *format, ...)
+int mp_snprintf_cat(char *str, size_t size, const char *format, ...)
     PRINTF_ATTRIBUTE(3, 4);
 
 struct bstr;

player/command.c

@@ -1561,7 +1561,7 @@ static int probe_deint_filters(struct MPContext *mpctx)
         if (pref > 0 && pref <= 4) {
             const char *types[] =
                 {"", "first-field", "bob", "temporal", "temporal-spatial"};
-            mp_snprintf_append(filter, sizeof(filter), ":deint-mode=%s",
+            mp_snprintf_cat(filter, sizeof(filter), ":deint-mode=%s",
                             types[pref]);
         }