Keep mouse inside 3D viewport rotation widget

Hide and keep the mouse in place when the user oribts the scene via the 3D
rotation widget.

(cherry picked from commit 051f02a3a0)

editor/plugins/spatial_editor_plugin.h

@@ -158,6 +158,7 @@ class ViewportRotationControl : public Control {
 	SpatialEditorViewport *viewport = nullptr;
 	Vector<Color> axis_colors;
 	Vector<int> axis_menu_options;
+	Vector2i orbiting_mouse_start;
 	bool orbiting = false;
 	int focused_axis = -2;