Decides what is considered a surface edge.

Values of this enumeration:

• HighToLow - Change from higher points to lower ones is considered a surface edge.
• LowToHigh - Change from lower points to higher ones is considered a surface edge.
• AnyLowHigh - Any change in points height is going to be considered a surface edge.
• InvalidToValid - Change from invalid points to valid ones is considered a surface edge.
• ValidToInvalid - Change from valid points to invalid ones is considered a surface edge.
• AnyValidInvalid - Any change in points validity is going to be considered a surface edge.