[engine] Decode animation frames with alpha after an opaque first frame - #193113

Fixes #85831

While working on webp encoder for image package i noticed that flutter does not render some animated webp and gifs with following exception:

======== Exception caught by image resource service ================================================
The following _Exception was thrown resolving an image frame:
Exception: Could not getPixels for frame 1

When the exception was thrown, this was the stack: 
Path: \\Mac\Downloads\gif\200w.webp
====================================================================================================

Code sample and visual demo with the files used, since the issue has no screenshots and author did not provide his code sample:

import 'dart:io';

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Material App',
      home: Scaffold(
        appBar: AppBar(title: Text('Material App Bar')),
        body: Center(child: Column(children: <Widget>[
          Image.file(File(r'\\Mac\Downloads\gif\200w.gif'), height: 100,),
          Image.file(File(r'\\Mac\Downloads\gif\200w.webp'), height: 100,),

          Image.file(File(r'\\Mac\Downloads\gif\giphy.gif'), height: 100,),
          Image.file(File(r'\\Mac\Downloads\gif\giphy.webp'), height: 100,),

          Image.file(File(r'\\Mac\Downloads\gif\opaque_first_frame_bug.gif'), height: 100,),
        ])),
      ),
    );
  }
}

Webp and gif Images used for this sample:

It appears that this is an old issue in how Flutter uses Skia, because for animated WebP and GIF, Skia's SkCodec::getInfo() reports the alpha type of the first frame only, and its documentation doesn't mention that (or at least i did not find it). Flutter allocates every frame's bitmap from that info without checking getFrameInfo(i).fAlphaType, so Skia rejects any later frame with alpha after an opaque first frame with kInvalidConversion.

Tested on an engine built from master (e619960, Windows x64 release) with an app that decodes every frame through ui.instantiateImageCodec:

Fix: when SkCodec::getInfo() reports kOpaque_SkAlphaType for an animation, BuiltinSkiaCodecImageGenerator now reports kPremul_SkAlphaType if any later frame reports a non-opaque SkCodec::FrameInfo::fAlphaType. Animations whose later frames all report kOpaque_SkAlphaType stay opaque. APNG is decoded by APNGImageGenerator, which this change doesn't touch.

The new tests in testing/dart/codec_test.dart use a 140-byte WebP and a 70-byte GIF fixture. I ran codec_test.dart with flutter_tester from a host_debug_unopt_x64 build (Windows x64) with the Skia and Impeller Vulkan backends, single and multithreaded: all 16 tests pass. Without the change the two new tests fail with Could not getPixels for frame 1. The Impeller OpenGL ES tests were not tested locally.

Screenshot after the fix, both with impeller enabled and disabled:

Pre-launch Checklist

• I read the Contributor Guide and followed the process outlined there for submitting PRs.
• I read the AI contribution guidelines and understand my responsibilities, or I am not using AI tools.
• I read the Tree Hygiene wiki page, which explains my responsibilities.
• I read and followed the Flutter Style Guide, including Features we expect every widget to implement.
• I signed the CLA.
• I listed at least one issue that this PR fixes in the description above.
• I updated/added relevant in-code documentation (doc comments with ///).
• If this PR introduces a new feature or capability, I created and linked a website documentation issue or PR in flutter/website (or verified none is needed).
• I added new tests to check the change I am making, or this PR is test-exempt.
• I followed the breaking change policy and added Data Driven Fixes where supported.
• All existing and new tests are passing.