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.

tests/opengl/data/general/multiview5

@@ -0,0 +1,26 @@
+puts "========"
+puts "0032886: Visualization, V3d_View - introduce interface for creating a subview"
+puts "V1/CompView + V1/ThumbView at corner"
+puts "========"
+
+pload MODELING VISUALIZATION
+vinit V1/CompView -width 380 -height 520 -composer 1
+vbackground GRAY20
+vaxo
+vcamera -persp
+box b 1 2 3
+vdisplay -dispMode 1 b
+vaspects b -faceBoundaryDraw 1
+vfit
+vzbufftrihedron
+
+vinit V1/ThumbView -parent V1/CompView -width 0.25 -height 0.25 -corner bottomRight -top 10 -left 10
+vbackground GRAY40
+vaxo
+vfit
+
+vactivate V1/CompView
+vdump $::imagedir/${::casename}.png
+
+vactivate V1/ThumbView
+vdump $::imagedir/${::casename}_thumb.png