Allow multiple objects to have the same mask

[dev7355608]

Description of change

Multiple objects having the same mask is possible, but the code doesn't seem to be written with that in mind. isMask can easily end up being false even if the object is still used as a mask. The current implementation can cause problems if the mask object is before the object that is masked in the scene graph: the mask is rendered normally for one frame after isMask is set to false and renderable to true when the mask is removed from another object; isMask remains false, but renderable is set to false again by the mask system. Here a demo where the the mask is added and removed from some other object every frame and so the mask is rendered normally in the background. https://www.pixiplayground.com/#/edit/jC73hmNsD59Hgyp6kseVL. If the mask is after the all objects that use the mask in the scene graph, this visual bug wouldn't occur; only isMask would be incorrectly false.

I fixed this problem by adding a reference counter that counts the number of times the object is used as a mask.

Pre-Merge Checklist

• Tests and/or benchmarks are included
• Documentation is changed or added
• Lint process passed (npm run lint)
• Tests passed (npm run test)

[bigtimebuddy]

Overall I don't have an issue with this, however, I think your tests should also cover destroying elements to make sure the mask refs are updated too.

[dev7355608]

Overall I don't have an issue with this, however, I think your tests should also cover destroying elements to make sure the mask refs are updated too.

Destroy slipped my mind. Fixed that.