Use malloc for Nan::NewBuffer owned data

GC results in free() rather than delete[]

Add plenty of new valgrind suppressions

test/leak/leak.sh

@@ -1,8 +1,18 @@
+#!/bin/sh
+
 if ! type valgrind >/dev/null; then
   echo "Please install valgrind before running memory leak tests"
   exit 1
 fi
 
-curl -O https://raw.githubusercontent.com/jcupitt/libvips/master/libvips.supp
-cd ../../
-G_SLICE=always-malloc G_DEBUG=gc-friendly valgrind --suppressions=test/leak/libvips.supp --suppressions=test/leak/sharp.supp --leak-check=full --show-leak-kinds=definite,indirect,possible --num-callers=20 --trace-children=yes npm test
+curl -o ./test/leak/libvips.supp https://raw.githubusercontent.com/jcupitt/libvips/master/libvips.supp
+
+G_SLICE=always-malloc G_DEBUG=gc-friendly valgrind \
+  --suppressions=test/leak/libvips.supp \
+  --suppressions=test/leak/sharp.supp \
+  --gen-suppressions=yes \
+  --leak-check=full \
+  --show-leak-kinds=definite,indirect,possible \
+  --num-callers=20 \
+  --trace-children=yes \
+  npm test