[Impeller] toImage and toImageSync produce differing pixel contents on Vulkan host tests #193029

On the Vulkan backend (both on Linux and on macOS host tests via MoltenVK), Picture.toImage and Picture.toImageSync produce differing pixel byte outputs for the same recorded canvas operations. This causes the test toImage and toImageSync have identical contents in testing/dart/canvas_test.dart to fail when executed with --impeller-backend=vulkan.

This issue tracks investigating and resolving the rendering or alpha/premultiplication discrepancies between both snapshot paths on Vulkan (replacing legacy closed issue #108835).

Steps to Reproduce

1. In engine/src/flutter/testing/dart/canvas_test.dart, remove the backend skip condition inside test('toImage and toImageSync have identical contents').
2. Run the Dart test runner with --impeller-backend=vulkan.

Observed Behavior

The byte data returned by toImage().toByteData() does not match toImageSync().toByteData().

Expected Behavior

Both asynchronous and synchronous snapshot methods should produce identical rasterized pixel buffers across all Impeller backends.

Environment

• Engine testing environment (testing/dart/canvas_test.dart)
• Impeller Vulkan backend (Linux and macOS MoltenVK)