[cairo] Runtime check for AlphaBlend (win32 backend)

[Tor Lillqvist]

Tim Rowley writes:
 > In mozilla we've found that AlphaBlend isn't available or is broken on
 > older versions of windows.  This patch adds a runtime check for the
 > function.

But Cairo uses other NT-only API, too: CreateFontIndirectW(),
GetGlyphOutlineW(), ModifyWorldTransform(), SetWorldTransform(), and
SetGraphicsMode(GM_ADVANCED), at least, so that AlphaBlend() change
won't make it run on Win98, as far as I can see. Is this just by
accident, or an deliberate design choice? If it is by choice, does
this mean HEAD GTK+ and GLib can forget about Win9x, too? (I sure
would hope so...)

--tml