Make egui work on WebGPU out of the box. (#2945)

* Make wgpu webgl/gles opt-in (but still work out of the box via feature flag), workaround canvas creation issue

* missing allow unsafe code annotations

* add breaking change not to eframe release notes

* Add --webgpu flag to build_demo_web.sh

* Improve CHANGELOG docs

* Clean up, and prepare for having to wasm:s

* put canvas without workaround under `if false`

* fix spelling

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Co-authored-by: Emil Ernerfeldt <emil.ernerfeldt@gmail.com>

crates/egui-wgpu/src/lib.rs

@@ -68,6 +68,8 @@ impl Default for WgpuConfiguration {
                 features: wgpu::Features::default(),
                 limits: wgpu::Limits::default(),
             },
+            // Add GL backend, primarily because WebGPU is not stable enough yet.
+            // (note however, that the GL backend needs to be opted-in via a wgpu feature flag)
             backends: wgpu::Backends::PRIMARY | wgpu::Backends::GL,
             present_mode: wgpu::PresentMode::AutoVsync,
             power_preference: wgpu::PowerPreference::HighPerformance,