add 8.6 compat and mitchell

Author: jcupitt

ChangeLog

@@ -1,6 +1,11 @@
 started 8.7.0 22/5/18
 - added vips7compat.h include for libvips 8.7
 - more output for -V to help debugging CLI mode
+- revised Text widget
+- added Canny
+- Sobel uses the new vips_sobel() operator
+- add mitchell kernel
+- add 8.6 compat 
 
 started 8.6.1 1/5/18
 - better enum display in header

configure.ac

@@ -381,6 +381,7 @@ AC_OUTPUT([
 	share/nip2/compat/8.3/Makefile
 	share/nip2/compat/8.4/Makefile
 	share/nip2/compat/8.5/Makefile
+	share/nip2/compat/8.6/Makefile
 	src/BITMAPS/Makefile 
 	src/Makefile
 	test/Makefile