LibWeb: Fix MouseEvent position values

[Psychpsyo]

The clientX and clientY values are, as per the spec, the offset from the viewport.
This makes them actually be that and also fixes up the calculations for offsetX, offsetY, pageX and pageY.
I assume all of these got messed up in some sort of refactor in the past.

The spec comment from the now-removed compute_mouse_event_client_offset() function sadly has no convenient place to be anymore so, for now, it is just gone as well.
Personally, I think it'd make sense to refactor a lot of this file so that not every mouse event repeats a large chunk of (almost) identical code. That way there'd be a nice place to put the comment without repeating it all over the file.
But that is out of the scope of this PR.

Also: I know, offsetX and Y are not fully fixed yet, they still don't ignore the element's CSS transforms but I am working on that in a new PR.

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[kalenikaliaksandr]

could you add a test for this change?

[Psychpsyo]

could you add a test for this change?

I've added a test over in #2372 (essentially part 2 of this commit) that takes care of all the mouse position values at once.
I hope that's ok.

[gmta]

I've added a test over in #2372 (essentially part 2 of this commit) that takes care of all the mouse position values at once. I hope that's ok.

I'd rather have a (partial) test in this PR so we don't depend on merging another PR for this code to be tested.

[Psychpsyo]

I've split the test into two now and added the relevant half here.

[kalenikaliaksandr]

could you include the text from the PR description in the commit message? the current commit message is a bit sparse, and since the commit message is more important than the PR description, it would be better to provide a detailed explanation there.

[Psychpsyo]

could you include the text from the PR description in the commit message? the current commit message is a bit sparse, and since the commit message is more important than the PR description, it would be better to provide a detailed explanation there.

Also done.