[Mesa-stable] cherry-pick request 9608193cb for 10.3 branch

[Roland Scheidegger]

The following commit should be added to 10.3 branch, otherwise mesa
won't compile with msvc, this addresses bug
https://bugs.freedesktop.org/show_bug.cgi?id=85918:

commit 9608193cbc6ea14e49adcd0193f9e7c6058d5a2f
Author: Brian Paul <brianp at vmware.com>
Date:   Wed Sep 10 08:16:24 2014 -0600

    mesa: fix UNCLAMPED_FLOAT_TO_UBYTE() macro for MSVC

    MSVC replaces the "F" in "255.0F" with the macro argument which leads
    to an error.  s/F/FLT/ to avoid that.

    It turns out we weren't using this macro at all on MSVC until the
    recent "mesa: Drop USE_IEEE define." change.

    Reviewed-by: Roland Scheidegger <sroland at vmware.com>

Roland