Fix issues with stroke depth ordering

[davepagurek]

Resolves #6386

On some machines, for some tests, lines are rendering behind fills in WebGL. This resolves the issue. More info in the comments on what specifically is involved.

[davepagurek]

• highp doesn't make a difference to the test failures but would address Line shaders not working on Chrome/Linux #6386 and prevent mobile issues
• It looks like Line.vert fix for small units #7206 isn't in the dev-2.0 branch, so I tried adding that. No effect, but it should be here anyway
• it seems the actual cause is modified stroke vertex shader #7064, specifically the line float zOffset = mix(-0.00045, -1., facingCamera);. It was pushing lines backwards, relying on it to round to the same value when facing the camera, but this breaks things if the depth precision is high enough or things round slightly differently.

[davepagurek]

The original issue that seems to have been causing this, #6956, has a test case where strokes were cutting through ortho cubes incorrectly due to the shift in depth of lines. Testing that same example again, the same issue is no longer present (I think thanks to #7206) even when I replace -0.00045 with 0. So I think this is all set!

I've added a new test case with a mini version of the original ortho cube issue in order to prevent regression.