Made GLX 1.3 a requirement for the xlib OpenGL port

Modern Unix like systems (GNU/Linux, etc...) just have to be kept
update. After all it is Free Software (aka. "Open Source") and not
some proprietary OS that forces us to drag along old workarounds for
backwards compatibillity because there is no way of changing it and
the developers wont.
Added OpenGL multi sample anti aliasing support
Added OpenGL multisample AA implementation to WinDOS backend
Added fixes for OpenGL stubs

Various functions that should replace OpenGL functions when
SGUI_NO_OPENGL is defined were outdated.
Added vsync setting on OpenGL windows
Updated README