Added mswin plat header.
mswin/ogl runs!

There's a strange interaction with fullscreen. If we create a window
that covers the whole desktop, it won't come back out of fullscreen.

If we create a window with a border, or one that has even one pixel
uncovered (like setting witdh to xd - 1), this doesn't happen. If we
cover the whole desktop with a WS_POPUP window, no other windows will
update while that window exists. We can't even hide the window.
Resizing, changing style flags, nothing lets other apps render again.
The alt-tab window and start menu don't even appear!

I'm farily certain this is an optimization somewhere. Windows detects
that the ogl window covers a whole monitor and gives it more resources,
but there isn't code to restore after.

The work around is likely to be to destroy the window and recreate it
but that puts a real damper on alt-tab. It means we can't show the
window in fullscreen during alt-tab?

Running wow -opengl mostly works correctly in windowed fullscreen.
Alt-tab works, though there is a black flash every time you enter/
exit the modal loop.

The internet isn't helping.