Fix a minor issue in StrumLine

[HEIHUAa]

Previously, using str.ID to read the state might have been inconsistent with the input from the code above:

for (i in 0...membersLength) {
    __pressed[i] = members[i].__getPressed(this);
    __justPressed[i] = members[i].__getJustPressed(this);
    __justReleased[i] = members[i].__getJustReleased(this);
}

It has now been unified to use the members index. Additionally, the new approach may offer better performance.