Add raw pixel data support to boolean and withOverlay ops

The previously-scattered image opening logic has been refactored to a
single ImageDescriptor struct/Object available to both JS and C++ code

This removed about 150 LOC but more importantly reduces the complexity
of adding/exposing new operations that require an input image.

src/metadata.h

@@ -2,6 +2,41 @@
 #define SRC_METADATA_H_
 
 #include "nan.h"
+#include "common.h"
+
+struct MetadataBaton {
+  // Input
+  sharp::InputDescriptor *input;
+  // Output
+  std::string format;
+  int width;
+  int height;
+  std::string space;
+  int channels;
+  int density;
+  bool hasProfile;
+  bool hasAlpha;
+  int orientation;
+  char *exif;
+  size_t exifLength;
+  char *icc;
+  size_t iccLength;
+  std::string err;
+
+  MetadataBaton():
+    input(nullptr),
+    width(0),
+    height(0),
+    channels(0),
+    density(0),
+    hasProfile(false),
+    hasAlpha(false),
+    orientation(0),
+    exif(nullptr),
+    exifLength(0),
+    icc(nullptr),
+    iccLength(0) {}
+};
 
 NAN_METHOD(metadata);