fix color average

[tonny008]

fixes #279

rOut = (rA * aA / 255) + (rB * aB * (255 - aA) / (255*255))
gOut = (gA * aA / 255) + (gB * aB * (255 - aA) / (255*255))
bOut = (bA * aA / 255) + (bB * aB * (255 - aA) / (255*255))
aOut = aA + (aB * (255 - aA) / 255)

[sksamuel]

Awesome thank you !

[sksamuel]

Looks like a test failure.

[tonny008]

com.sksamuel.scrimage.core.ScalingDeadlockTest > image scale should not deadlock on multiple concurrent scales FAILED
    io.kotest.assertions.AssertionFailedError: expected:<50> but was:<49>
        at app//com.sksamuel.scrimage.core.ScalingDeadlockTest$1$1.invokeSuspend(ScalingDeadlockTest.kt:30)
        at app//com.sksamuel.scrimage.core.ScalingDeadlockTest$1$1.invoke(ScalingDeadlockTest.kt)
        at app//com.sksamuel.scrimage.core.ScalingDeadlockTest$1$1.invoke(ScalingDeadlockTest.kt)

val images = mutableListOf<ImmutableImage>() is it thread safe?

[sksamuel]

Ah yeah I guess not.