Testing blend modes with Unity's shaders

"Screen" blending does not seem to be possible.
Tweaked background parameters to work better with new shader.
Merge branch 'shader-test'
Reorganized all assets - top level is now system, not asset type

Structure previously was:
Scripts/lightning, utilities, etc.

Structure now is:
Lighting/Scripts ; Generic/Scripts/Utilities
Moved physics from LightningBall to GameBall.

Lightning should contain generic scripts and prefabs, physics should not be assumed.
Render background first, fixing sprites. Updated web build.