Add convolve operation for kernel-based convolution (#479)

src/pipeline.h

@@ -1,6 +1,8 @@
 #ifndef SRC_PIPELINE_H_
 #define SRC_PIPELINE_H_
 
+#include <memory>
+
 #include <vips/vips8>
 
 #include "nan.h"
@@ -83,6 +85,11 @@ struct PipelineBaton {
   std::string err;
   bool withMetadata;
   int withMetadataOrientation;
+  std::unique_ptr<double[]> convKernel;
+  int convKernelWidth;
+  int convKernelHeight;
+  double convKernelScale;
+  double convKernelOffset;
   int tileSize;
   int tileOverlap;
   VipsForeignDzContainer tileContainer;
@@ -136,6 +143,10 @@ struct PipelineBaton {
     optimiseScans(false),
     withMetadata(false),
     withMetadataOrientation(-1),
+    convKernelWidth(0),
+    convKernelHeight(0),
+    convKernelScale(0.0),
+    convKernelOffset(0.0),
     tileSize(256),
     tileOverlap(0),
     tileContainer(VIPS_FOREIGN_DZ_CONTAINER_FS),