set _renderComponent is null in mask disable (cocos#2961)

Video player current time display error (cocos#2951)

Merge branch 'v1.9'

Merge branch 'v1.9-release'

Fix issues (cocos#2608)

* Add missed vec3 APIs

* Fix Node property setter issue

* Fix scene dirty sync issue

* Fix setWorldRot not setting dirty issue

update version to 1.6.0