Full green screen (#4790)
Full green screen (#4790)

* hide gradient in greenscreen

* support for full screen + green screen

switchinnovations/pxt,playi/pxt,switchinnovations/pxt,switch-education/pxt,switch-education/pxt,switchinnovations/pxt,switch-education/pxt,Microsoft/pxt,switchinnovations/pxt,switch-education/pxt,Microsoft/pxt,Microsoft/pxt,switch-education/pxt,playi/pxt,playi/pxt,Microsoft/pxt,playi/pxt,Microsoft/pxt