screenshots: create screenshot dir

Minor user convenience.

player/screenshot.c

@@ -286,6 +286,8 @@ static char *gen_fname(screenshot_ctx *ctx, const char *file_ext)
             dir = mp_get_user_path(t, ctx->mpctx->global, dir);
             fname = mp_path_join(NULL, bstr0(dir), bstr0(fname));
             talloc_free(t);
+
+            mp_mkdirp(dir);
         }
 
         if (!mp_path_exists(fname))