Forest ground too dark when pillaring up

There are two closely related issues:

• When pillaring upwards in a forest, the light levels below the tree decrease as soon as you reach a new, formerly empty section. (To better see this, disable "Smooth lighting")
• When light reaches an empty section, the same happens

For a demonstration, see: https://youtu.be/KQ--QM2s4l0
For a fix, see: https://github.com/MinecraftForge/MinecraftForge/pull/3870