This is necessary since non-base OpenGL functions can't be connected by
calling the standard GetProcAddress() on Windows. To archive this the
old qwglGetProcAdress() is renamed into GetProcAdressGL() and a new
function pointer qwglGetProcAdress is introduced. On unixoid System it's
NULL and on Windows connected to wglGetProcAddress(). If it's NULL the
system wide function is called, if not the function Pointer is used.
To archive this, 3 new functions Sys_GetProcAddress(), Sys_LoadLibrary()
and Sys_FreeLibrary() were added to abstract the library loading code
into a platform independend API.
