Until now only the class Texture used it. Now it has been added to ElementArrayBuffer, ArrayBuffer, FrameBuffer and ShaderProgram.
Static class members have to be initialized outside the header, so the file ElementArrayBuffer.cc had to be added.
Unbind calls e.g. glBindBuffer(GL_ARRAY_BUFFER, 0) outside of the specific class are not useful anymore, because it
would not reset the static context variable and thus lead to bugs/crashes.