0032886: Visualization, V3d_View - introduce interface for creating a subview

V3d_View/Graphic3d_CView pair has been extended to define subview within the other V3d_View instance.
The initialization is done in form of V3d_View::SetWindow() taking parent V3d_View instance on input.

Subview definition includes dimensions defined as a fraction of a parent view and offset from a corner.
This scheme allows splitting window into several subviews automatically occupying entire viewport,
like splitting window into two vertial subviews (100%x50% + 100%x50%),
three horizontal subviews (33%x100% + 30%x100% + 30%x100%),
1 + 2 stacked subviews (50%x100% + 50%x50% + 50%x50%),
as well as thumbnail-alike subviews displayed on top of another larger view.

OpenGl_View::Redraw() blits content of subviews into the window within immediate redraw step.

AIS_ViewController::FlushViewEvents() has been extended
to re-calculate mouse input into local subview coordinates.
AIS_ViewController::handleViewRedraw() first redraws subviews and then parent views.
Introduced new callback AIS_ViewController::OnSubviewChanged()
to switch input focus to another subview on mouse click,
implemented by ViewerTest_EventManager (has to be done at application level).

vinit command has been extended with parameters -subview and -parent to create a subview.
In addition, view dimension arguments now can be defined as a fraction of screen size instead of pixels.

src/ViewerTest/ViewerTest_EventManager.hxx

@@ -79,6 +79,9 @@ public:
     myPickPntArgVec[2] = theArgZ;
   }
 
+  //! Update mouse scroll event.
+  Standard_EXPORT virtual bool UpdateMouseScroll (const Aspect_ScrollDelta& theDelta) Standard_OVERRIDE;
+
   //! Handle mouse button click event.
   Standard_EXPORT virtual bool UpdateMouseClick (const Graphic3d_Vec2i& thePoint,
                                                   Aspect_VKeyMouse theButton,
@@ -116,6 +119,12 @@ public:
   //! Handle KeyPress event.
   Standard_EXPORT void ProcessKeyPress (Aspect_VKey theKey);
 
+  //! Callback called on Selection of another (sub)view.
+  //! This method is expected to be called from rendering thread.
+  Standard_EXPORT virtual void OnSubviewChanged (const Handle(AIS_InteractiveContext)& theCtx,
+                                                 const Handle(V3d_View)& theOldView,
+                                                 const Handle(V3d_View)& theNewView) Standard_OVERRIDE;
+
 protected:
 
   //! Register hot-keys for specified Action.