wgpu backend: Enable WebGL fallback (#8038)

* Fix for https://github.com/emilk/eframe_template/issues/223
* Related: https://github.com/gfx-rs/wgpu/pull/9319

By default, we would only turn on the WebGPU backend on web, which means
browsers without WebGPU support would just crash.

You can still opt-out of all the default `wgpu` features by enabling
`eframe/wgpu_no_default_features` instead of `eframe/wgpu`

Cargo.lock

@@ -1330,7 +1330,6 @@ dependencies = [
  "wasm-bindgen",
  "wasm-bindgen-futures",
  "web-sys",
- "wgpu",
 ]
 
 [[package]]