Limit HEIF output dimensions to 16384x16384

This is a slightly breaking change to sync with the behaviour of a
forthcoming libvips patch release. It also matches the libavif
limit. Having an arbitrary limit is safer than no limit.

test/unit/avif.js

@@ -130,4 +130,18 @@ describe('AVIF', () => {
       width: 32
     });
   });
+
+  it('Invalid width - too large', async () =>
+    assert.rejects(
+      () => sharp({ create: { width: 16385, height: 16, channels: 3, background: 'red' } }).avif().toBuffer(),
+      /Processed image is too large for the HEIF format/
+    )
+  );
+
+  it('Invalid height - too large', async () =>
+    assert.rejects(
+      () => sharp({ create: { width: 16, height: 16385, channels: 3, background: 'red' } }).avif().toBuffer(),
+      /Processed image is too large for the HEIF format/
+    )
+  );
 });