Sprite's containsPoint method check alpha

Hello!

`Sprite` support `containsPoint` method, But it just check the point is in the sprite area depend on width and height not by imageData.

For example, if the sprite image is a totally transparent image, `containsPoint` should always return false.

My thought maybe like this

1. extract `imageData` from `app.stage`

2. check the pixel relative to the point is alpha

Any better advise to do this?

Thank you!