Fix: still track mouse when dragging outside web canvas

[emilk]

• Closes Web demo loses mouse cursor position when dragging off the window #3157

If the mouse leaves the canvas when dragging a slider, the slider will still move.

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To support this, I had to revert #4419
Despite that, I fail to reproduce the two issues it claimed to solve:

• Scrollbars and other draggable elements get stuck mouse leaves browser window #4406 may have been solved in another way by this PR
• Not detecting mouse release after alt-tab on windows #4418 I cannot reproduce on Mac. If it is still a problem, I think it should be solved by triggering a PointerEvent::Released when focus is lost (i.e. on alt-tab), and not on PointerGone

[jprochazk]

This works, but it behaves a bit strangely when dragging windows. The window always moves by the mouse's delta, including if the mouse is outside the viewport.

1. Open the web demo
2. Start dragging a window
3. Move the mouse outside of the viewport, e.g. into the browser title bar
4. Move the mouse back into the viewport

Now the mouse is outside of the window it's dragging, but the mouse movement still moves the window. I think one possible fix would be to not apply any mouse delta for a given axis if the mouse is outside of the canvas clientRect on that axis.

For example: Canvas client rect is top_left=[100 100], bottom_right=[200 200], mouse is x=100, y=50, moving the mouse to x=100, y=70 will not move the window 20 pixels in +y.

[emilk]

@jprochazk I think the problem there is the drag-tracking in Window should just store where on the Window the drag started (in local coordinates). That is: Window dragging should be based on the absolute position, not relative deltas. In fact, very few things should be based on drag delta

[jprochazk]

Nice!