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.

test/unit/boolean.js

@@ -40,6 +40,23 @@ describe('Boolean operation between two images', function() {
         });
     });
 
+    it(op + ' operation, raw', function(done) {
+      sharp(fixtures.inputJpgBooleanTest)
+        .raw()
+        .toBuffer(function(err, data, info) {
+          if (err) throw err;
+          sharp(fixtures.inputJpg)
+            .resize(320, 240)
+            .boolean(data, op, { raw: info })
+            .toBuffer(function(err, data, info) {
+              if (err) throw err;
+              assert.strictEqual(320, info.width);
+              assert.strictEqual(240, info.height);
+              fixtures.assertSimilar(fixtures.expected('boolean_' + op + '_result.jpg'), data, done);
+            });
+        });
+    });
+
   });
 
   it('Invalid operation', function() {

test/unit/overlay.js

@@ -528,4 +528,28 @@ describe('Overlays', function() {
       });
   });
 
+  it('Composite RGBA raw buffer onto JPEG', function(done) {
+    sharp(fixtures.inputPngOverlayLayer1)
+      .raw()
+      .toBuffer(function(err, data, info) {
+        if (err) throw err;
+        sharp(fixtures.inputJpg)
+          .resize(2048, 1536)
+          .overlayWith(data, { raw: info })
+          .toBuffer(function(err, data) {
+            if (err) throw err;
+            fixtures.assertSimilar(fixtures.expected('overlay-jpeg-with-rgb.jpg'), data, done);
+          });
+      });
+  });
+
+  it('Throws an error when called with an invalid file', function(done) {
+    sharp(fixtures.inputJpg)
+      .overlayWith('notfound.png')
+      .toBuffer(function(err) {
+        assert(err instanceof Error);
+        done();
+      });
+  });
+
 });