Update libpng 1.6.22beta

Includes SSE filter optimizations that will reduce decode runtime
by about 30% on Intel CPUs.

BUG:27264258
Change-Id: Ia7b9aeaddb43381c71e40607b5ccb40ebd3f5a31
diff --git a/scripts/makefile.openbsd b/scripts/makefile.openbsd
index ef5be81..fea4a7a 100644
--- a/scripts/makefile.openbsd
+++ b/scripts/makefile.openbsd
@@ -11,7 +11,7 @@
 MANDIR= ${PREFIX}/man/cat
 
 SHLIB_MAJOR=	16
-SHLIB_MINOR=	1.6.20
+SHLIB_MINOR=	1.6.22beta03
 
 LIB=	png
 SRCS=	png.c pngerror.c pngget.c pngmem.c pngpread.c \