Add background option to tile output operation (#1924)

lib/constructor.js

@@ -213,6 +213,7 @@ const Sharp = function (input, options) {
     tileSize: 256,
     tileOverlap: 0,
     tileSkipBlanks: -1,
+    tileBackground: [255, 255, 255, 255],
     linearA: 1,
     linearB: 0,
     // Function to notify of libvips warnings

lib/output.js

@@ -551,6 +551,7 @@ function toFormat (format, options) {
  * @param {Number} [options.size=256] tile size in pixels, a value between 1 and 8192.
  * @param {Number} [options.overlap=0] tile overlap in pixels, a value between 0 and 8192.
  * @param {Number} [options.angle=0] tile angle of rotation, must be a multiple of 90.
+ * @param {String|Object} [options.background={r: 255, g: 255, b: 255, alpha: 1}] - background colour, parsed by the [color](https://www.npmjs.org/package/color) module, defaults to white without transparency.
  * @param {String} [options.depth] how deep to make the pyramid, possible values are `onepixel`, `onetile` or `one`, default based on layout.
  * @param {Number} [options.skipBlanks=-1] threshold to skip tile generation, a value 0 - 255 for 8-bit images or 0 - 65535 for 16-bit images
  * @param {String} [options.container='fs'] tile container, with value `fs` (filesystem) or `zip` (compressed file).
@@ -603,6 +604,8 @@ function tile (options) {
         throw is.invalidParameterError('angle', 'positive/negative multiple of 90', options.angle);
       }
     }
+    // Background colour
+    this._setBackgroundColourOption('tileBackground', options.background);
     // Depth of tiles
     if (is.defined(options.depth)) {
       if (is.string(options.depth) && is.inArray(options.depth, ['onepixel', 'onetile', 'one'])) {