[scripting] Fix effect.animate() curve argument being actually used
One cannot use a non metatype frrom an external class inside an
End result is the script engine arguments wouldn't match up and MOC
would just use the default value.
As far as I can tell this has been broken for 6 years.
The global animate method that unboxes a QJSValue as an object is
No shipped kwin effect actually used it.
To some extent we didn't even actually want to enforce the enum as we
also accept custom value of ScriptedEffect::GuassianCurve, so it has
been switched for an int.
Test Plan: Unit test
Reviewers: KWin, broulik
Reviewed By: broulik
Differential Revision: https://phabricator.kde.org/D14669