screenshot: use mp_stripext for %F

Reuse existing functionality, see previous commit introducing above
function.

Author: WhitePeter

player/screenshot.c

@@ -68,14 +68,6 @@ void screenshot_init(struct MPContext *mpctx)
     };
 }
 
-static char *stripext(void *talloc_ctx, const char *s)
-{
-    const char *end = strrchr(mp_basename(s), '.');
-    if (!end)
-        end = s + strlen(s);
-    return talloc_asprintf(talloc_ctx, "%.*s", (int)(end - s), s);
-}
-
 static bool write_screenshot(struct mp_cmd_ctx *cmd, struct mp_image *img,
                              const char *filename, struct image_writer_opts *opts,
                              bool overwrite)
@@ -179,7 +171,7 @@ static char *create_fname(struct MPContext *mpctx, char *template,
             char *name = video_file ? mp_basename_or_url(video_file) : "NO_FILE";
 
             if (fmt == 'F')
-                name = stripext(res, video_file);
+                name = mp_stripext(res, name);
             append_filename(&res, name);
             break;
         }