mesa: use F_TO_I() instead of IROUND()

mesa: use F_TO_I() instead of IROUND()

Use it where performance matters more and the exact method of float->int
conversion/rounding isn't terribly important.  There should no net change
here since F_TO_I() is the new name of the old IROUND() function.

Reviewed-by: José Fonseca <jfonseca@vmware.com>