[web] Fix multi-view sizing race condition (State isolation approach) - #185624

Summary

This PR resolves the multi-view sizing regression reported in #185034. The issue stemmed from multiple views sharing a single OffscreenSurface instance. Concurrent or interleaved rendering calls would cause the surface to be resized for one view while another view was still expecting its own dimensions.

Design

This approach isolates the rendering state by giving every OffscreenCanvasViewRasterizer its own dedicated OffscreenSurface. By moving the surface ownership from the Rasterizer (singleton-like) to the ViewRasterizer (per-view), we eliminate contention and the possibility of cross-view interference.

Pros:

• Better performance as views do not need to wait for each other to render.
• Architecturally cleaner state management for multi-view.

Cons/Risks:

• Increased memory usage per view.
• Risk of hitting browser WebGL context limits if the user opens a large number of views (e.g., >10).

Testing

• Verified via new test engine/src/flutter/lib/web_ui/test/ui/multi_view_sizing_test.dart.