js: Use correct autoptr in gjs_string_to_filename()

The return value of g_filename_from_utf8() (must be freed with g_free())
was getting assigned to a GjsAutoJSChar (which would be freed with
JS_free()). Instead gjs_string_to_filename() must have a GjsAutoChar
output parameter.

This requires changing gjs_parse_call_args() as well. We improve it by
allowing to pass in autoptrs for 's' or 'F' args instead of char**.

Fixes a few minor memory leaks, mostly in error paths in Cairo functions.

https://bugzilla.gnome.org/show_bug.cgi?id=785040