screenshots: create screenshot dir

Minor user convenience.

DOCS/man/options.rst

@@ -2661,8 +2661,9 @@ Screenshot
     generated by ``--screenshot-template``. If the template filename is already
     absolute, the directory is ignored.
 
-    If the directory does not exist or is not a directory, an error is
-    generated.
+    If the directory does not exist, it is created on the first screenshot. If
+    it is not a directory, an error is generated when trying to write a
+    screenshot.
 
     This option is not set by default, and thus will write screenshots to the
     directory from which mpv was started. In pseudo-gui mode

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))