Add mixed and minSize animation options for WebP output

docs/api-output.md

@@ -291,6 +291,8 @@ Use these WebP options for output image.
     *   `options.effort` **[number][12]** CPU effort, between 0 (fastest) and 6 (slowest) (optional, default `4`)
     *   `options.loop` **[number][12]** number of animation iterations, use 0 for infinite animation (optional, default `0`)
     *   `options.delay` **([number][12] | [Array][13]<[number][12]>)?** delay(s) between animation frames (in milliseconds)
+    *   `options.minSize` **[boolean][10]** prevent use of animation key frames to minimise file size (slow) (optional, default `false`)
+    *   `options.mixed` **[boolean][10]** allow mixture of lossy and lossless animation frames (slow) (optional, default `false`)
     *   `options.force` **[boolean][10]** force WebP output, otherwise attempt to use input format (optional, default `true`)
 
 ### Examples

docs/changelog.md

@@ -8,6 +8,8 @@ Requires libvips v8.13.0
 
 * Drop support for Node.js 12, now requires Node.js >= 14.15.0.
 
+* Add WebP `minSize` and `mixed` options for greater control over animation frames.
+
 * Use combined bounding box of alpha and non-alpha channels for `trim` operation.
   [#2166](https://github.com/lovell/sharp/issues/2166)