1) Not every transform's internal signal is designed to fit in 16 bits.
2) If overflow happens in this function, it indicates that we need to
adjust the txfm's scaling. We shouldn't mute the overflow signal.
3) Saturation might be handy when all of our transform design are stable,
but I don't think we are at the stable point yet.
4) This will fix C/Trans16x16DCT.AccuracyCheck/1 failure in highbd mode.

Change-Id: I5ef5d130c22adb4b8c3b608ffcb0f2c99dc7523f