Core: Remove artist-friendly factor of PI from shaders

[WestLangley]

This PR removes the artist-friendly factor of PI from the shaders, and moves it into the renderer, instead.

Now, the factor is computed once per frame instead of once per fragment.

Previously, MeshLambertMaterial ignored the .physicallyCorrectLights flag, so when the flag was set, MeshLambertMaterial rendered more brightly than MeshPhongMaterial by a factor of PI.

Only users who use MeshLambertMaterial while setting the .physicallyCorrectLights flag to true will see a change.

[WestLangley]

Instead, we probably should have had a different flag -- something like "Artist-Friendly Lights". Such a flag would be true by default.

Setting the .physicallyCorrectLights flag to true is not sufficient to make the lights physically-correct, anyway. The light .decay must be set to 2, world units must be in meters, and then light intensities must be specified in the proper units.

[mrdoob]

So much nicer! 😍

[mrdoob]

Many thanks!