Fix dragging sprites

[nimeratus]

Resolves

• Fixes Bug when dragging overlapping sprites #378

Proposed Changes

• Remove the data about which sprite is dragged from the <Stage>'s React state and put it in its JavaScript properties
• Sort the candidate drawables into layer order in RenderWebGL.pick()

Reason for Changes

• Prevents the situation where it starts dragging a sprite but then the info that it's already dragging something doesn't update before it gets a second mouse event, and so it starts dragging another sprite but doesn't tell the first one that it isn't being dragged anymore
• Makes sure that the topmost sprite is picked up when dragging

Test Coverage

I've exported player.html and player.js and imported this project into it and tried to reproduce the bugs and they didn't happen

[github-actions]

Thank you for your submission, we really appreciate it. Like many open-source projects, we ask that you sign our Contributor License Agreement before we can accept your contribution. You can sign the CLA by just posting a Pull Request Comment same as the below format.

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I have read the CLA Document and I hereby sign the CLA

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[nimeratus]

I have read the CLA Document and there was no "not" before the "infringe".
This way that sentence means things like "You warrant that the Contribution will infringe upon the rights of any third party."

[rosics-code]

Looks good!