Fix coplanar strokes in WebGL mode not respecting depth + pixels[] being flipped

[davepagurek]

Resolves #2938
Also resolves #5386 because I noticed it while writing tests and needed it for my test to work :')

Changes:

• Makes retained and immediate WebGL modes consistent by drawing strokes after fills
• Adjusts the stroke shader to draw at the same depth as fills when perpendicular to the camera (and smoothly transitioning to being slightly in front of fills as they face away from the camera)
• Makes get(x,y) and pixels consistent with 2D mode (previously, y was flipped)

Screenshots of the change:

Before:

After:

Live: https://editor.p5js.org/davepagurek/sketches/aSFkpcttn

Live demo of 3D shapes: https://editor.p5js.org/davepagurek/sketches/JBttrCMV-

PR Checklist

• npm run lint passes
• [Inline documentation] is included / updated
• [Unit tests] are included / updated

[davepagurek]

@aferriss if you get the chance, does this approach seem ok to you? (I kind of forgot about this change for a while, but it'd be great to address the underlying issue in the next p5 release!)

[aferriss]

Apologies, I missed this! I see you added these changes to the updatePixels PR though so maybe we can just close this and merge in the changes over there.

[davepagurek]

going to close this one, the changes were merged in with #6109 :)