replace IROUND with util functions

replace IROUND with util functions

This adds two new util functions to rounding.h, _mesa_iroundf and
mesa_lround, which are just wrappers around roundf and round, that cast
to int and long int respectively. This is possible since mesa recently
dropped support for VC2013, since 2015 and 2017 support roundf.

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>