Flip Y , make the result be natural.

[finscn]

Before change:

After change:

In most real scenes , when the container is a top-short & bottom-long trapezoid , the children also should be "top-short & bottom-long".
Like this :

So I think this PR is necessary.

[finscn]

I know we could use factor=-1 to flip Y ,
but if use factor=-1 , when we move squareY , the result is not as same as expected

[finscn]

I see. I could change the example code like this:

app.ticker.add(function (delta) {
    // now we can get local coords for points of perspective
    let posX = container.toLocal(squareX.position, undefined, undefined, undefined, PIXI.projection.TRANSFORM_STEP.BEFORE_PROJ);
    container.proj.setAxisX(posX, squareX.factor);

    let posY = container.toLocal(squareY.position, undefined, undefined, undefined, PIXI.projection.TRANSFORM_STEP.BEFORE_PROJ);
    posY.y = -posY.y;
    posY.x = -posY.x;
    container.proj.setAxisY(posY, -squareY.factor);
});

But If use this PR , the code

    posY.y = -posY.y;
    posY.x = -posY.x;
    container.proj.setAxisY(posY, -squareY.factor);

could be removed .
just leave one line :

 container.proj.setAxisY(posY, squareY.factor);

the code will be clear.