Fix CVE-2016-5296: Heap-buffer-overflow WRITE in rasterize_edges_1
Adapted for upstream pixman based on:

  https://hg.mozilla.org/releases/mozilla-esr45/rev/5e39c1c2fded

--- pixman-0.34.0/pixman/pixman-edge-imp.h.orig	2015-06-30 05:48:31.000000000 -0400
+++ pixman-0.34.0/pixman/pixman-edge-imp.h	2016-11-16 01:09:34.046335106 -0500
@@ -55,8 +55,9 @@
 	 *
 	 * (The AA case does a similar  adjustment in RENDER_SAMPLES_X)
 	 */
-	lx += X_FRAC_FIRST(1) - pixman_fixed_e;
-	rx += X_FRAC_FIRST(1) - pixman_fixed_e;
+	/* we cast to unsigned to get defined behaviour for overflow */
+	lx = (unsigned)lx + X_FRAC_FIRST(1) - pixman_fixed_e;
+	rx = (unsigned)rx + X_FRAC_FIRST(1) - pixman_fixed_e;
 #endif
 	/* clip X */
 	if (lx < 0)
