Improve Screen Position to be more intuitive

[diogovtx]

Screen Position was historically based off Unity's own ComputeScreenPos(), which is poorly named. "Screen Position" meant Unity's NDC space, not screen space.

My proposal is to keep current functionality, to avoid breaking current shaders, but extend it to expose both normalized viewport UV and pixel position outputs directly, similar to UE's ScreenPosition node.

[Dawie3565]

Note will need to update documentation also.

[Dawie3565]

for ComputeScreenPo see *Compute Screen Pos Node https://wiki.amplify.pt/index.php?title=Unity_Products:Amplify_Shader_Editor/Compute_Screen_Pos

for Screen Position Node see action item #144