swscale: fix overflow in gray16 vertical scaling.

This fixes the same overflow as in the RGB48/16-bit YUV scaling;
some filters can overflow both negatively and positively (e.g.
spline/lanczos), so we bias a signed integer so it's "half signed"
and "half unsigned", and can cover overflows in both directions
while maintaining full 31-bit depth.

Signed-off-by: Mans Rullgard <mans@mansr.com>
