Use existing ID3D11Texture2D or OpenGL texture directly in Engine

[Rama]

@hesham;187042

I’ve created a pull request from my branch and welcome comments or improvements (eg: OpenGL support or fixing compiling on non-OpenGL/Direct3D platforms):

https://github.com/EpicGames/UnrealEngine/pull/650

Thank you for your contribution Hesham!

Rama