The alpha value was not being shifted, and because of associativity, the
alpha color was also not being inverted before being divided by 255.

Found while doing other Color work.

This was introduced in
    commit a0842e53
    Date:   Mon May 12 05:22:52 2014 +0200
    more sal_Int32 to sal_uInt32 and simplification

Change-Id: I99b9e866a4f63676e194e65d64c31fb4c0442e0a
Reviewed-on: https://gerrit.libreoffice.org/50245Tested-by: Jenkins <ci@libreoffice.org>
Reviewed-by: Noel Grandin <noel.grandin@collabora.co.uk>