Refactor internal 'resize' to more apt 'pipeline'

Refactor 'composite' C to C++ 'operations'

src/operations.h

@@ -0,0 +1,24 @@
+#ifndef SRC_OPERATIONS_H_
+#define SRC_OPERATIONS_H_
+
+namespace sharp {
+
+  /*
+    Composite images `src` and `dst` with premultiplied alpha channel and output
+    image with premultiplied alpha.
+   */
+  int Composite(VipsObject *context, VipsImage *src, VipsImage *dst, VipsImage **out);
+
+  /*
+   * Premultiply alpha channel of `image`.
+   */
+  int Premultiply(VipsObject *context, VipsImage *image, VipsImage **out);
+
+  /*
+   * Unpremultiply alpha channel of `image`.
+   */
+  int Unpremultiply(VipsObject *context, VipsImage *image, VipsImage **out);
+
+}  // namespace sharp
+
+#endif  // SRC_OPERATIONS_H_