0033607: Visualization - Implementation of hardware occlusion queries

[Hossam86]

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Hardware occlusion queries provide a mechanism to determine whether any pixels would be drawn. For instance, it can be used as a visibility filter any pixels would be drawn. for instance, it can be used as a visibility filter

Graphic3d_ViewOcclusionMask : Provides a way to access occlusion test results for every graphical presentation per each defined view of the viewer

OpenGl_OcclusionQuery: Provides an OpenGL implementation for occlusion queries

Graphic3d_RenderingParams: provides way to enable or disable occlusion query

AIS_InteractiveContext : resets the view occlusion mask for newly loaded objects

Each view invalidates its occlusion query results if the camera has changed.

vnboccluded: Adds a new DRAW command to return the number of occluded objects in the view