Replace IROUND_POS with _mesa_roundevenf

Replace IROUND_POS with _mesa_roundevenf

Which has the same behavior as long as you don't change the FPU rounding
mode. Other code in mesa makes the same assumption so it should be safe
to make that assumption more generally.

Reviewed-by: Marek Olšák <marek.olsak@amd.com>
Reviewed-by: Kristian H. Kristensen <hoegsberg@google.com>
Reviewed-by: Matt Turner <mattst88@gmail.com>
Part-of: <https://gitlab.freedesktop.org/mesa/mesa/-/merge_requests/3024>