Add pdfBackground constructor property (#4207)

lib/constructor.js

@@ -139,6 +139,7 @@ const debuglog = util.debuglog('sharp');
  * @param {number} [options.page=0] - Page number to start extracting from for multi-page input (GIF, WebP, TIFF), zero based.
  * @param {number} [options.subifd=-1] - subIFD (Sub Image File Directory) to extract for OME-TIFF, defaults to main image.
  * @param {number} [options.level=0] - level to extract from a multi-level input (OpenSlide), zero based.
+ * @param {string|Object} [options.pdfBackground] - Background color to use when PDF is partially transparent. Parsed by the [color](https://www.npmjs.org/package/color) module to extract values for red, green, blue and alpha. Requires the use of a globally-installed libvips compiled with support for PDFium, poppler-glib, ImageMagick, or GraphicsMagick.
  * @param {boolean} [options.animated=false] - Set to `true` to read all frames/pages of an animated image (GIF, WebP, TIFF), equivalent of setting `pages` to `-1`.
  * @param {Object} [options.raw] - describes raw pixel input image data. See `raw()` for pixel ordering.
  * @param {number} [options.raw.width] - integral number of pixels wide.
@@ -348,6 +349,7 @@ const Sharp = function (input, options) {
     timeoutSeconds: 0,
     linearA: [],
     linearB: [],
+    pdfBackground: [255, 255, 255, 255],
     // Function to notify of libvips warnings
     debuglog: warning => {
       this.emit('warning', warning);