Minor wgpu-web releated stuff (#1977)

* egui_demo_app: remove wgpu and pollster as direct dependencies

* eframe: use same web-sys version as wgpu crate

* Make note that web_sys_unstable_apis is required by the wgpu crate

* Rename the glow web painter in eframe

* Remove trait DummyWebGLConstructor from web_glow_painter.rs

* cargo fmt

* Fix check.sh

Cargo.lock

@@ -1257,12 +1257,10 @@ dependencies = [
  "ehttp",
  "image",
  "poll-promise",
- "pollster",
  "serde",
  "tracing",
  "tracing-subscriber",
  "tracing-wasm",
- "wgpu",
 ]
 
 [[package]]
@@ -4260,9 +4258,9 @@ dependencies = [
 
 [[package]]
 name = "web-sys"
-version = "0.3.59"
+version = "0.3.58"
 source = "registry+https://github.com/rust-lang/crates.io-index"
-checksum = "ed055ab27f941423197eb86b2035720b1a3ce40504df082cac2ecc6ed73335a1"
+checksum = "2fed94beee57daf8dd7d51f2b15dc2bcde92d7a72304cdf662a4371008b71b90"
 dependencies = [
  "js-sys",
  "wasm-bindgen",