Fix stacking order of canvas

[axelboc]

Regression introduced in #1387 ... so much for bringing stacking under control 😓

The canvas was still stacked implicitly at z-index: 0, so in front of the grid and SVG elements... Sorry about that. So now it moves behind everything.

Unfortunately, in doing so, r3fRoot—which is still stacked implicitly at z-index: 0 (and cannot be stacked explicitly without creating a new stacking context and breaking the stacking order, as explained in a comment in the CSS)—captures events before the reach the canvas... I had to work around this by moving pointer-events: none to r3fRoot instead of setting it on every overlay, and then re-enabling it to auto on the canvas and floating toolbar.

This feels better in the end, since it removes a bunch of pointer-events: none and saves us and consumers from forgetting to add them.

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Note that this may very well change when dealing with #1388. To be continued...

[loichuder]

Thanks!