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For certain types of filters where the intermediate sum of coefficients can go above the fixed-point equivalent of 1.0 in the middle of a filter, the sum of a 31-bit calculation can overflow in both directions and can thus not be represented in a 32-bit signed or unsigned integer. To work around this, we subtract 0x40000000 from a signed integer base, so that we're halfway signed/unsigned, which makes it fit even if it overflows. After the filter finishes, we add the scaled bias back after a shift. We use the same trick for 16-bit bpc YUV output routines. Signed-off-by: Mans Rullgard <mans@mansr.com>