average method
Average the alpha, red, green, and blue channels
of a Color with another other.
The resultant alpha may be overridden by providing a strength.
See alphaFromStrength for details on using a value between 0..1 or
2..255 as a shortcut for specifying alpha/opacity override.
Exposure method for ~/ operator, providing strength.
Implementation
Color average(Color other, [dynamic strength]) =>
withAlpha(alphaFromStrength(strength) ?? alpha) ~/ other;