gfxlib2: Fix Win32 OpenGL driver to set fullw/fullh variables properly

http://www.freebasic.net/forum/viewtopic.php?f=3&t=21419

Commit 5b3523fd exposed this problem with the Win32 OpenGL driver:
it initialized the window to 8x8 and only resized it afterwards;
so the real size wasn't stored into fb_win32.fullw/fullh, but the
WM_GETMINMAXINFO handler expects to be able to rely on those.