Landscape weight-blended layers don't paint exclusively anymore

1. Create a landscape and setup 2 or more weight-blended target layers (say Red, Green and Blue).
2. Fill the landscape with one of the layers (say, Red). 
3. Paint some other layer on top of it (e.g. Green)

In 5.6 : painting Green will progressively replace the Red layer (i.e. the Green layer will be increased and the Red, decreased) : [Link Removed]

In 5.7 : each layer is painted independently : the Green layer will be increased but the Red layer will not be decreased, leading to a blended color at the intersection : [Link Removed]

While technically correct (the layers' weights are normalized such that their sum is always one), this feels unnattural to long-time users since it now requires "un-painting" the other weight-blended layers, which can be tedious. 

Better tools are coming in UE5.8 but in the meantime, we need a solution for UE5.7 users