Expose private members of ArcballCameraControl as protected

[kephale]

We need to modify these variables in subclasses, so they should be protected instead of private.

scenery/src/main/kotlin/graphics/scenery/controls/behaviours/ArcballCameraControl.kt

Lines 33 to 35 in 6b2aa7d

	private val logger by lazyLogger()
	private var lastX = w / 2
	private var lastY = h / 2