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

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Drop the following outdated API methods conflicting with Z-layers API: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.