Add skipBlanks support for tile layout (#1687)

src/pipeline.h

@@ -172,6 +172,7 @@ struct PipelineBaton {
   VipsForeignDzLayout tileLayout;
   std::string tileFormat;
   int tileAngle;
+  int skipBlanks;
   VipsForeignDzDepth tileDepth;
   std::unique_ptr<double[]> recombMatrix;
 
@@ -271,6 +272,7 @@ struct PipelineBaton {
     tileContainer(VIPS_FOREIGN_DZ_CONTAINER_FS),
     tileLayout(VIPS_FOREIGN_DZ_LAYOUT_DZ),
     tileAngle(0),
+    skipBlanks(-1),
     tileDepth(VIPS_FOREIGN_DZ_DEPTH_LAST) {}
 };