The modes available to measure VisualElement sizes.
Additional resources: TextElement.MeasureTextSize
| Undefined | The element should give its preferred width/height without any constraint. |
| Exactly | The element should give the width/height that is passed in and derive the opposite site from this value (for example, calculate text size from a fixed width). |
| AtMost | At Most. The element should give its preferred width/height but no more than the value passed. |