Add mixed and minSize animation options for WebP output

lib/constructor.js

@@ -251,6 +251,8 @@ const Sharp = function (input, options) {
     webpNearLossless: false,
     webpSmartSubsample: false,
     webpEffort: 4,
+    webpMinSize: false,
+    webpMixed: false,
     gifBitdepth: 8,
     gifEffort: 7,
     gifDither: 1,

lib/output.js

@@ -469,6 +469,8 @@ function png (options) {
  * @param {number} [options.effort=4] - CPU effort, between 0 (fastest) and 6 (slowest)
  * @param {number} [options.loop=0] - number of animation iterations, use 0 for infinite animation
  * @param {number|number[]} [options.delay] - delay(s) between animation frames (in milliseconds)
+ * @param {boolean} [options.minSize=false] - prevent use of animation key frames to minimise file size (slow)
+ * @param {boolean} [options.mixed=false] - allow mixture of lossy and lossless animation frames (slow)
  * @param {boolean} [options.force=true] - force WebP output, otherwise attempt to use input format
  * @returns {Sharp}
  * @throws {Error} Invalid options
@@ -506,6 +508,12 @@ function webp (options) {
         throw is.invalidParameterError('effort', 'integer between 0 and 6', effort);
       }
     }
+    if (is.defined(options.minSize)) {
+      this._setBooleanOption('webpMinSize', options.minSize);
+    }
+    if (is.defined(options.mixed)) {
+      this._setBooleanOption('webpMixed', options.mixed);
+    }
   }
   trySetAnimationOptions(options, this.options);
   return this._updateFormatOut('webp', options);