Fix selectable label highlight offset when alignment is changed

[termlt]

Description:

Fixes the offset issue with the selectable label highlight when the label text alignment is changed.

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Screenshot of the issue:

---

Test application to replicate the issue:

package main

import (
	"fyne.io/fyne/v2"
	"fyne.io/fyne/v2/app"
	"fyne.io/fyne/v2/widget"
)

func main() {
	a := app.New()
	w := a.NewWindow("Test")
	w.Resize(fyne.NewSize(700, 600))

	test := widget.NewLabel("Test")
	test.Alignment = fyne.TextAlignCenter
	test.Selectable = true

	w.SetContent(test)

	w.ShowAndRun()
}

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Checklist:

• Tests all pass.

[Jacalz]

Thanks for the PR. I left a few general comments inline but I leave a more thorough review of the label code to someone with more experience in that part of the codebase.

I would like to add that you please don't remove the PR template. The checkboxes are there for a reason. Please add them back and try to tick off the first three mandatory ones. It would be good to add a test case for this to verify that the change fixes that issue. It also seems like this breaks the existing tests so that would need to be fixed as well.

[Jacalz]

Please format the inputs correctly using goimports.

[Jacalz]

Doesn't this change break the set alignment for anything other than leading?

[Jacalz]

I'm not 100% well versed in the label and text code but I kind of feel like there already should be logic somewhere in the code to calculate things like this.

[andydotxyz]

The existing selection code should have most of this maths already - it's the leading X which changes - and that should just be the existing width calculations subtracted from the width?

[andydotxyz]

I'm pretty sure this only accounts for selection of a single line but multi-line text can be selected as well