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:

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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 included.
• Lint and formatter run with no errors.
• Tests all pass.

[andydotxyz]

Fantastic thanks for fixing this up.

However I think it only works for a single line label / selection.
The complete fix probably needs to iterate each line in a selection instead of just all selections in a single move operation.