Upgrade to libvips v8.6.0

Expose offset coordinates of strategy-based crop
Switch to Github releases for prebuilt libvips
Move packaging scripts to lovell/sharp-libvips repo

src/pipeline.h

@@ -62,8 +62,9 @@ struct PipelineBaton {
   int channels;
   Canvas canvas;
   int crop;
-  int cropCalcLeft;
-  int cropCalcTop;
+  bool hasCropOffset;
+  int cropOffsetLeft;
+  int cropOffsetTop;
   bool premultiplied;
   std::string kernel;
   std::string interpolator;
@@ -145,8 +146,9 @@ struct PipelineBaton {
     channels(0),
     canvas(Canvas::CROP),
     crop(0),
-    cropCalcLeft(-1),
-    cropCalcTop(-1),
+    hasCropOffset(false),
+    cropOffsetLeft(0),
+    cropOffsetTop(0),
     premultiplied(false),
     centreSampling(false),
     flatten(false),