Add withGainMap to process HDR JPEGs with embedded gain map #4314

lib/input.js

@@ -607,6 +607,7 @@ function _isStreamInput () {
  * - `tifftagPhotoshop`: Buffer containing raw TIFFTAG_PHOTOSHOP data, if present
  * - `formatMagick`: String containing format for images loaded via *magick
  * - `comments`: Array of keyword/text pairs representing PNG text blocks, if present.
+ * - `gainMap.image`: HDR gain map, if present, as compressed JPEG image.
  *
  * @example
  * const metadata = await sharp(input).metadata();