0026149: Visualization - depth buffer should not be written within Z-layers without Graphic3d_ZLayerDepthWrite flag

OpenGl_Workspace::updateMaterial() - do not activate writing into Depth buffer without Graphic3d_ZLayerDepthWrite flag.
Add method OpenGl_Workspace::UseDepthWrite() to track glDepthMask() state.

Drop the following outdated API methods conflicting with Z-layers API:
- V3d_View::EnableDepthTest(), V3d_View::IsDepthTestEnabled()
- Visual3d_View::ZBufferIsActivated(), Visual3d_View::SetZBufferActivity(), Visual3d_View::EnableDepthTest(), Visual3d_View::IsDepthTestEnabled()
- Graphic3d_GraphicDriver::SetDepthTestEnabled(), Graphic3d_GraphicDriver::IsDepthTestEnabled()

Activate Z-buffer by default, and manage it state only by Z-layer flags.

src/Graphic3d/Graphic3d_CView.hxx

@@ -37,7 +37,6 @@ public:
     ZClipBackPlane  (0.0f),
     DepthFrontPlane (0.0f),
     DepthBackPlane  (0.0f),
-    ZBufferActivity (0),
     Model (0),
     Visualization (0),
     NbActiveLight (0),
@@ -63,7 +62,6 @@ public:
   float DepthFrontPlane;
   float DepthBackPlane;
 
-  int   ZBufferActivity;
   int   Model;
   int   Visualization;
 

src/Graphic3d/Graphic3d_GraphicDriver.cdl

@@ -513,15 +513,6 @@ is
             is deferred;
         ---Purpose: call_togl_backfacing
 
-        SetDepthTestEnabled( me; view : CView from Graphic3d;
-                                 isEnabled : Boolean from Standard )
-    is deferred;
-    ---Purpose: call_togl_depthtest
-
-        IsDepthTestEnabled( me; view : CView from Graphic3d )
-    returns Boolean from Standard is deferred;
-    ---Purpose: call_togl_isdepthtest
-
         ReadDepths( me;
                     view          : CView from Graphic3d;
                     x, y          : Integer;