Add unflatten operation to create an alpha channel (#3461)

lib/operation.js

@@ -405,6 +405,25 @@ function flatten (options) {
   return this;
 }
 
+/**
+ * Unflatten - add an alpha channel to the image if required, and make white pixels fully transparent. Alpha for non-white pixels will be unchanged/opaque.
+ *
+ * @example
+ * await sharp(rgbInput)
+ *   .unflatten()
+ *   .toBuffer();
+ *
+ * @example
+ * await sharp(rgbInput)
+ *   .threshold(128, { grayscale: false }) // converter bright pixels to white
+ *   .unflatten()
+ *   .toBuffer();
+ */
+function unflatten (options) {
+  this.options.unflatten = is.bool(options) ? options : true;
+  return this;
+}
+
 /**
  * Apply a gamma correction by reducing the encoding (darken) pre-resize at a factor of `1/gamma`
  * then increasing the encoding (brighten) post-resize at a factor of `gamma`.
@@ -875,6 +894,7 @@ module.exports = function (Sharp) {
     median,
     blur,
     flatten,
+    unflatten,
     gamma,
     negate,
     normalise,