Workaround duplicate rendered characters in Electron 2.0.x

[bpasero]

This is a PR for electron-2.0.x branch (not master) with an attempt to workaround #52196.

We suspect the issue lies within Skia and thus hope is little to get this backported to Electron. As a workaround, putting those span elements with opacity on a separate drawing layer seems to fix this issue. This impacts:

• our unused variables style
• decorations from extensions that leverage opacity

[Tyriar]

I think we should merge this and here's why:

• It's temporary; we can revert it on the Electron 3 branch.

• While new layers are created for elements containing will-change, this only happens for .squiggly-inline-unnecessary which are on unused variables (and decorations). In a typical project this should be fairly minimal.

• It's far less risky that the alternative, the only "damage" it could do AFAIK is lower frame rate.

• Even with a really bad case like the screenshot below (61 elements with .squiggly-inline-unnecessary) where the entire screen is filled with an unused variable, I can barely perceive a change in FPS.

  

[alexdima]

👍 LGTM

[alexdima]

test