Cesium3DTileset's "Enable Water Mask" option has a decoy checkbox

[kring]

I couldn't figure out why disabling the water mask wasn't working. Until I realized I was unchecking the first box, which is... well, I have no idea what it is. But I know now that it doesn't actually disable the feature. To disable that water mask, you have to keep that one checked, and uncheck the rightmost one.

We should figure out where this dodgy decoy checkbox is coming from and get rid of it.

[azrogers]

I believe this is a result of the EditCondition meta flag on the EnableWaterMask UPROPERTY, as detailed here. I think the first box edits the boolean that EditCondition points to (which is bIsMac in this case), whereas the second box actually changes the value. What a bizarre feature.

We can probably get rid of this by either adding EditConditionHides to hide the property if on Mac, or by creating a CanEditChange method to disable it instead.