Win32 optimizing compiler causes color loss in gdImageCopyResampled()

Hello,

Great library!  I found that with MS Visual Studio 8 (compiler version
14.00.50727.42) with global optimizations on (/O2) and libgd release 2.0.35,
there is an issue with the gdImageCopyResampled() in gd/gd.c.  Apparently
the MS compiler in its optimization causes a weird bug where it seems color
information is while scaling the image.  I was able to work around this bug
by using floor() instead of the (long) cast in the floor2() macro directly
above the function.  See the attached patch.

Best,
Keith