A new command vbackground is created. Old background commands are made aliases for the newly created command (including vsetdefaultbg).
Tests are modified using newly added command vbackground.
Graphic3d_CLight is now defined as a class inheriting Standard_Transient,
so that it's fields now should be accessed through methods.
Graphic3d_CLight::IsEnabled() - new property allowing to disable light source everywhere.
Confusing alias OpenGl_Light has been removed.
Graphic3d_CLight::SetAttenuation() - the upper limit 1.0 of attenuation factors has been removed
since it contradicts to OpenGL specs and does not make sense.
Graphic3d_ZLayerSettings::Lights() - light sources list is now property of ZLayer.
When defined, it overrides light sources defined for View/Viewer.
New class Graphic3d_LightSet has been defined to define a set of light sources.
V3d_Light - removed obsolete interface for debug drawing lights sources.
V3d_Light is now an alias to Graphic3d_CLight.
V3d_TypeOfLight is now defined as a typedef to Graphic3d_TypeOfLightSource.
