mesa: Replace F_TO_I() with _mesa_lroundevenf().

mesa: Replace F_TO_I() with _mesa_lroundevenf().

I'm not sure what the true meaning of "The rounding mode may vary." is,
but it is the case that the IROUND() path rounds differently than the
other paths (and does it wrong, at that).

Like _mesa_roundeven{f,}(), just add an use _mesa_lroundeven{f,}() that
has known semantics.

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