libobs-opengl: Terminate visual attribs with None

(Jim) The documentation for glXChooseFBConfig states that the last value
of the visual attributes array must be 'None'.  Fixes potential
initialization issues with certain drivers.

Closes jp9000/obs-studio#486

libobs-opengl/gl-x11.c

@@ -61,6 +61,7 @@ static int ctx_visual_attribs[] = {
 	GLX_BUFFER_SIZE, 32,
 	GLX_DOUBLEBUFFER, true,
 	GLX_X_RENDERABLE, true,
+	None
 };
 
 struct gl_windowinfo {