Dark tree leaves in 1.18

[devendrn]

1st picture: Caves and Cliffs enabled
2nd picture: Caves and Cliffs disabled

Heres the shadow map:

The tree leaves that appear dark no longer have color.a=0.0, instead they have color.a=1.0 now. As a result, the color.a method of tree leaves detection is no longer suitable.

An alternative tree leaves detection is needed to fix it.

[devendrn]

The issue has been partially fixed in c6a51a0