Clips away portions of the given heightmap, according to the Heightmap.Layer.Coverage layer of the given heightmap.
The coverage values of the resulting heightmap are computed like this:
inverse == false : c_out = c_in * c_mask inverse == true : c_out = in * (1 - c_mask)
c_out is the normalized coverage value (i.e. in the range [0..1]) of the resulting heightmap,
c_in is the normalized coverage value of first and
c_arg is the normalized coverage value of second.
The heightmap which Heightmap.Layer.Coverage layer to use for clipping portions of first.