Draw crosshair and pointSize consistently regardless the zoom level #930

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Run time

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Job	Run time
build (macos-latest, 3.9, pyqt5)	2m 45s
build (macos-latest, 3.9, pyside2)	2m 5s
build (ubuntu-latest, 3.9, pyqt5)	4m 59s
build (ubuntu-latest, 3.9, pyside2)	1m 37s
build (windows-latest, 3.9, pyqt5)	5m 38s
build (windows-latest, 3.9, pyside2)	3m 37s
	20m 41s