Ensure info.pages is correct when limiting input page range (#4578)

Author: metsw24-max

src/common.cc

@@ -711,9 +711,12 @@ namespace sharp {
     bool hasDelay = !delay.empty();
     VImage copy = image.copy();
 
-    // Only set page-height if we have more than one page, or this could
-    // accidentally turn into an animated image later.
-    if (nPages > 1) copy.set(VIPS_META_PAGE_HEIGHT, pageHeight);
+    // Only set page-height and n-pages if we have more than one page, or this
+    // could accidentally turn into an animated image later.
+    if (nPages > 1) {
+      copy.set(VIPS_META_PAGE_HEIGHT, pageHeight);
+      copy.set(VIPS_META_N_PAGES, nPages);
+    }
     if (hasDelay) {
       if (delay.size() == 1) {
         // We have just one delay, repeat that value for all frames.

test/unit/gif.js

@@ -64,6 +64,14 @@ suite('GIF input', () => {
     t.assert.strictEqual(80, info.pageHeight);
   });
 
+  test('Animated GIF limited to fewer pages than the input', async (t) => {
+    t.plan(3);
+    const { info } = await sharp(fixtures.inputGifAnimated, { pages: 2 }).toBuffer({ resolveWithObject: true });
+    t.assert.strictEqual(160, info.height);
+    t.assert.strictEqual(80, info.pageHeight);
+    t.assert.strictEqual(2, info.pages);
+  });
+
   test('GIF with reduced colours, no dither, low effort reduces file size', async (t) => {
     t.plan(1);
     const original = await sharp(fixtures.inputJpg)