Split the Output folder into Positive and Negative tests (KhronosGroup#508)

* Add two sub folders to Output, Positive and Negative. 

* Create separate manifest and validator results summary.

* List the contents separately in the main readme.

* Update the screenshot generator and validator scripts to handle the new output paths.

* Add a short explanation of Positive vs Negative to the Output folder.

* Save raw validator results in a folder with the relevant model group.

* Stop marking models as invalid in the manifest (affects only Primitive Restart).

Author: stevk