Tree Shading

For beta 1.7.3 and STAPI

Leaves rendering with shading mimicking b1.8+. Based on 'Adventure Update Tree Rendering' by NFC Vivian, but ported to StationAPI with simple Mixin.

Tested on clear Stapi instance and Beta Unleashed 2.0-beta.19.2.

Works with SmoothBeta and Macula.

It's not whole light engine rework, like newer versions did it, but small change to brighness of leaves only.