Add custom field validators for build

[hi-liang]

It's possible to input bad values into certain fields despite valid types. For instance, rotation takes number types but should be rangebound -- terms greater than 1 will mess up the scale of an object.

This seems like we can write some custom validators https://github.com/json-editor/json-editor#custom-validation

[mwfarb]

Using the standard json schema also works here: e4ab61c