[AD] GDI fix

["Matthew Leverton" <meffer@xxxxxxxxxx>]

To sum it up, there is a problem with the GDI driver blitting a
rectangle smaller than the bitmap's height that touches the bottom
left corner of a bitmap. The attached patch fixes it. (Only tested
under Vista.)

Note that it's no fault of Allegro. The StretchDIBits is just messed
up when using top-down bitmaps.

See <http://www.allegro.cc/forums/thread/592201> for a thread about it.

See <http://wiki.allegro.cc/StretchDIBits> for a detailed explanation.

--
Matthew Leverton

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