mesa: reimplement IROUND(), add F_TO_I()

mesa: reimplement IROUND(), add F_TO_I()

The different implementations of IROUND() behaved differently and in
the case of fistp, depended on the current x86 FPU rounding mode.
This caused some tests like piglit roundmode-pixelstore and
roundmode-getintegerv to fail on 32-bit x86 but pass on 64-bit x86.

Now IROUND() always rounds to the nearest integer (away from zero).
The new F_TO_I function converts a float to an int by whatever means
is fastest.  We'll use this where we're more concerned with performance
and not too worried to how the conversion is done.

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