alphaChannel static method
Considers the provided number(?) strength.
If strength is between 0..1, the return is an appropriate int for
the alpha channel representing that strength as an opacity multiplied by
the max channel value 0xFF.
After that check, if strength is an int or double, the return is
strength.
If all else fails, the return is null; say, if provided strength
itself is also null.
Implementation
static int? alphaChannel(num? strength) => alphaFromStrength(strength);