Move GetEngineFactoryXXXType and LoadGraphicsEngineXXX to Diligent namespace

[hals1010]

Small issue - the typedefs GetEngineFactoryXXXType and the functions LoadGraphicsEngineXXX are not part of the Diligent namespace.

This leads to code like

Diligent::EngineD3D11Attribs DeviceAttribs;
GetEngineFactoryD3D11Type pfnEngineFactoryD3D11;

LoadGraphicsEngineD3D11(pfnEngineFactoryD3D11);

instead of

Diligent::EngineD3D11Attribs DeviceAttribs;
Diligent::GetEngineFactoryD3D11Type pfnEngineFactoryD3D11;

Diligent::LoadGraphicsEngineD3D11(pfnEngineFactoryD3D11);

Best
Jörn