Landscape flatten target value and terrace interval's sliders don't take the landscape scale into account

This problem was originally reported in this UDN : https://udn.unrealengine.com/s/question/0D5QP000007yVue0AE/flatten-brush-limit-with-landscape-scale-z?fromCase=1

• Open the OpenWorld map
• Select the landscape actor and use a bigger scale value (e.g. 300)
• Open landscape mode (Shift+2)
• In the Sculpt panel, activate the Flatten tool
• Check the check box next to "Flatten Target"
• Use the slider 

This value (and it's the same for "Terrace Interval") is in world space and should be between the min and the max Z that the landscape can represent. With such a scale, we should therefore be able to go under -32768 and above 32768 units. However, the slider's min and max value are hardcoded to these 2 values and therefore the user cannot use a flatten target (or a terrace interval) that is outside that interval. 

Expected result : the user should be able to use a value that can get up to the maximum that the landscape can do and down to the minimum that it can do and these should be the bounds of the slider.