Support opt-in winit OffscreenCanvas worker windows

[eordano]

Allows Bevy's winit integration to use a canvas attached to a dedicated worker. Adds a disabled-by-default web-worker feature and skips page-only canvas lookup and parent sizing when the worker owns the canvas.

Five files, 21 additions and two deletions. The existing native and non-worker paths remain unchanged. Depends on robtfm/winit#1; the Cargo patch currently pins its public eordano/winit revision and should move to the merged revision when available.

Validation:

• Native, ordinary Wasm, and atomics Wasm bevy_winit checks pass with the feature both disabled and enabled.
• A real Bevy 0.16 WebGPU app handles keyboard input and resize on a dedicated worker. During a one-second page stall it advances six GPU submissions and six presented frames. During a 700 ms worker stall, page timers and button events remain responsive.
• Full upstream Decentraland Explorer Wasm build passes with these Bevy and winit dependencies.

This draft covers the engine integration. Moving Explorer's own bootstrap and browser APIs into a worker is separate application work.