Use windows.h directly for mingw32 build

Use windows.h directly for mingw32 build

This patch adresses the issue discussed in
http://lists.freedesktop.org/archives/pixman/2010-April/000163.html

There were only two clashing identifiers.  The first one is IN, which
obviously causes problems in Pixman for lines like

    PIXMAN_STD_FAST_PATH (IN, solid, a8, a8, fast_composite_in_n_8_8),

Fortunately the mingw headers provide a solution: by defining
_NO_W32_PSEUDO_MODIFIERS, these stupid symbols are skipped.

The other name is UINT64, used in pixman-mmx.c. I renamed that
function to to_uint64, but may be another name is more appropriate.