win32: Fix compilation of 'cairo-path-stroke-traps.c' with MSVC8

"The issue here is that Visual Studio 2005+ is quite strict on type
conversions (so it wants code to be clear enough on conversions)."

Fixes: https://bugs.freedesktop.org/show_bug.cgi?id=84908
(Cherry-pick of 56da7adc3c85be0325481ecd23746dc49e60d239 from master)
Merge branch '1.14' of git://anongit.freedesktop.org/git/cairo into v1-14-stable