Improve Pen tool handle dragging with Tab swapping, Ctrl angle locking, and directly dragging the closing endpoint in handle

[0SlowPoke0]

Partly closes #1870

Added features:

• Holding Ctrl while closing a path now locks the end handle to the angle of the opposite handle of the endpoint.

When dragging the handles of the segment being placed (pointer is down):

• Tab swaps to dragging the opposite handle (native: teleports the cursor / web: enters pointer lock and draws a fake cursor at the new location)
• If the current segment is closing the shape by connecting to its start point, the handle that's being dragged should be the closing handle, not a fake handle on the opposite side, and all the above shortcuts should work on that anchor/handle group spanning the start/end point (Tab will modify the shape of the first segment while the current/final segment is still in-progress)

Other issues fixed:

• When joining paths, switching between them, and clicking the handles of joined endpoints, the opposite handles now correctly:

  first.mp4

• Handles that appeared colinear but were not when using the path tool and dragging are now displayed correctly.

• When dragging a handle from the start of the segment and aborting the GRS it moves the end handle of segment:

  second.mp4

• Fixed this bug: https://files.keavon.com/-/PeachpuffEveryAfricanporcupine/capture_80_.mp4

[Keavon]

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[Keavon]

• Drawing a brand new path with the Pen tool, when I'm dragging from the initial point in my first mousedown, shouldn't let Tab work because there is no opposite handle to swap to.

• Holding Ctrl while closing a path now locks the end handle to the angle of the opposite handle of the endpoint.

  This has a few edge cases, here's a webcast with audio explaining them.

[Keavon]

!build

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[Keavon]

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[Keavon]

• We still have an unstable handle direction issue. I hold Ctrl before dragging out from the endpoint anchor but it's unstable when hovering within snapping distance of the anchor:

  capture_9_.mp4

• When clicking an endpoint to extend it, I see we now correctly use R to rotate the colinear pair. But then after using R several times repeatedly, it just seems to randomly break the colinearity without doing anything different from the previous several times:

  capture_8_.mp4

[Keavon]

!build

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