Fix incorrectly large reported height for animated gifs

img.js

@@ -415,6 +415,15 @@ class Image {
       metadata.height = width;
     }
 
+    if(metadata.pageHeight) {
+      // When the { animated: true } option is provided to sharp, animated
+      // image formats like gifs or webp will have an inaccurate `height` value
+      // in their metadata which is actually the height of every single frame added together.
+      // In these cases, the metadata will contain an additional `pageHeight` property which
+      // is the height that the image should be displayed at.
+      metadata.height = metadata.pageHeight;
+    }
+
     for(let outputFormat of outputFormats) {
       if(!outputFormat || outputFormat === "auto") {
         throw new Error("When using statsSync or statsByDimensionsSync, `formats: [null | auto]` to use the native image format is not supported.");

test/test.js

@@ -912,6 +912,7 @@ test("Animated gif", async t => {
 
   t.is(stats.gif.length, 1);
   t.is(stats.gif[0].width, 400);
+  t.is(stats.gif[0].height, 400);
   // it’s a big boi
   t.true( stats.gif[0].size > 1000*1000 );
 });