[Freeciv-Dev] (PR#12173) win32: alpha blending support

["James Canete" <use_less@xxxxxxxxxxx>]

<URL: http://bugs.freeciv.org/Ticket/Display.html?id=12173 >

After much headscratching, trial and error, and cursing the limitations
of GDI, here is a patch that implements alpha blending in gui-win32.

It currently has the following problems:

1) Doesn't work in Windows 95.
2) Doesn't work in 16 bit color.

To get it working on either would require a move to a different API, or
writing code that would mix colors directly, neither idea I am willing
to entertain. :) I haven't yet created code to deal with these
situations, so if you test it in such a situation, expect explosions. :)

Included in this patch is a little cleanup I did of the win32 version of
the canvas struct, which allows a canvas to point at a window handle, as
well as the bitmap and device context it could handle before.

And now I sleep. :)
-James Canete

win32-alpha.diff
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