gfxlib2: Use more unique name for Win32 window class

to prevent parallel gfxlib instances from re-using the window class created
by the first one (e.g. if Screen was used in the .exe and in a DLL it uses).

This is needed because the window class defines the message handling
function used for the graphics window. If a DLL were to re-use the .exe's
window class, it would also use the .exe's fb_hWin32WinProc(), instead
of its own fb_hWin32WinProc(). Then all the keypresses and
WM_GETMINMAXINFO requests etc. would be processed in the wrong gfxlib
instance.

See also: http://www.freebasic.net/forum/viewtopic.php?p=190961#p190961