Add support for proper grass removal

Previously, the "RemoveGrass" paint layers did not work well with landscape "Edit Layers". So, it didn't work well for Landsmass or the Water plugin.

Watch this tutorial: https://youtu.be/YHYYFpLc968

This version fixes those issues and no we can simply paint "RemoveGrass" to remove grass at any place. (Even inside various "Edit Layers"). We can also use "RemoveGrass" as a paint layer inside Landmass & as a weightmap layer inside the Water System.

In addition to that, we fixed #15.