cmake build: added 64Bit detection under linux
some little housekeeping
fixed build error when using SmartRefCont<T> var(<pointer to intance of T>)
added an option to the OpenGLRenderer to indicate that an existing Context should be used instead of creating an own one.
This option is used to integrate the Unrimp OpenGL Renderer into an QtQuick2/Qml2 application (using Render to Texture)
see https://github.com/thewolfwillcome/qt-unrimp for an example
added OpenGLRenderer_SingleHeader.h which combines all Texture related definitions of the OpenGL Renderer into one Header (similar to the Renderer.h file)
This Header is used by the qt-unrimp example to get the OpenGL Texture Id. (In qt-unrimp the renderer renders into a texture)