fix(with_winit): enable wgpu backends so web demo loads

[matthargett]

Summary

• The hosted with_winit web demo has been crashing on startup in every browser (Firefox, Chrome, Safari) with RuntimeError: unreachable inside wgpu::Instance::new, leaving a blank page.
• Root cause: the workspace declares vello = { ..., default-features = false } (since vello_svg itself doesn't need wgpu), which drops vello's wgpu_default feature. The with_winit example only re-enabled vello's bare wgpu feature, so wgpu was compiled with no backends — and on wasm32 its Instance::new panics with "No wgpu backend feature that is implemented for the target platform was enabled."
• Fix: enable vello's wgpu_default feature (instead of wgpu) for the with_winit example on all targets, so the webgpu backend (and the native backends) are actually compiled in.

Verified locally with cargo build -p with_winit --bin with_winit_bin --target wasm32-unknown-unknown. cargo tree -p wgpu --target wasm32-unknown-unknown now shows wgpu features include webgpu, where previously none of the backend features were enabled.

Fixes #80

Test plan

• CI passes
• After merge, visit https://linebender.github.io/vello_svg/ in Firefox / Chrome / Safari and confirm the SVG scenes render instead of a blank page with a console panic