Fix deprecated onSceneGUIDelegate warning for Unity 2019.1 or newer

[rfadeev]

Summary

Fixed warnings when using package via UPM with Unity 2019.1 or newer:

Packages\NormalPainter\Assets\Editor\NormalPainterWindow.cs(38,13): warning CS0618: 'SceneView.onSceneGUIDelegate' is obsolete: 'onSceneGUIDelegate has been deprecated. Use duringSceneGui instead.'
Packages\NormalPainter\Assets\Editor\NormalPainterWindow.cs(43,13): warning CS0618: 'SceneView.onSceneGUIDelegate' is obsolete: 'onSceneGUIDelegate has been deprecated. Use duringSceneGui instead.'

If needed, fine for me to open pull request to dev branch.

Details

Unity made onSceneGUIDelegate obsolete in 2019.1, see https://github.com/Unity-Technologies/UnityCsReference/blob/2019.1/Editor/Mono/SceneView/SceneView.cs#L363. In 2018.4 it was not obsolete, see https://github.com/Unity-Technologies/UnityCsReference/blob/2018.4/Editor/Mono/SceneView/SceneView.cs#L253. Used Unity version define to support both versions.

How to test

Install package via UPM using source branch and Unity 2018.4 and Unity 2019.1 and check normal painter editor window works:

"com.utj.normalpainter": "https://github.com/rfadeev/NormalPainter.git#fix-onsceneguidelegate-warning",