swscale: fix overflows in output of RGB48 pixels.

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>

libswscale/swscale.c

@@ -754,8 +754,8 @@ yuv2rgb48_X_c_template(SwsContext *c, const int16_t *lumFilter,
 
     for (i = 0; i < (dstW >> 1); i++) {
         int j;
-        int Y1 = 0;
-        int Y2 = 0;
+        int Y1 = -0x40000000;
+        int Y2 = -0x40000000;
         int U  = -128 << 23; // 19
         int V  = -128 << 23;
         int R, G, B;
@@ -771,7 +771,9 @@ yuv2rgb48_X_c_template(SwsContext *c, const int16_t *lumFilter,
 
         // 8bit: 12+15=27; 16-bit: 12+19=31
         Y1 >>= 14; // 10
+        Y1 += 0x10000;
         Y2 >>= 14;
+        Y2 += 0x10000;
         U  >>= 14;
         V  >>= 14;