Bug 1799748 - Backport fix for libpixman CVE-2022-44638. r=jfkthame

Backport of upstream commit:
a1f88e842e

Differential Revision: https://phabricator.services.mozilla.com/D161624

gfx/cairo/libpixman/src/pixman-trap.c

@@ -74,7 +74,7 @@ pixman_sample_floor_y (pixman_fixed_t y,
 
     if (f < Y_FRAC_FIRST (n))
     {
-	if (pixman_fixed_to_int (i) == 0x8000)
+	if (pixman_fixed_to_int (i) == 0xffff8000)
 	{
 	    f = 0; /* saturate */
 	}