Update to wgpu 0.20 (#4560)

* this PR reverts https://github.com/emilk/egui/pull/4559
* and re-applies https://github.com/emilk/egui/pull/4433

Before we merge, we're waiting for a wgpu 0.20.1 patch-release of

* https://github.com/gfx-rs/wgpu/pull/5681

---------

Co-authored-by: Andreas Reich <r_andreas2@web.de>

crates/egui_demo_app/src/apps/custom3d_wgpu.rs

@@ -49,11 +49,13 @@ impl Custom3d {
                 module: &shader,
                 entry_point: "vs_main",
                 buffers: &[],
+                compilation_options: wgpu::PipelineCompilationOptions::default(),
             },
             fragment: Some(wgpu::FragmentState {
                 module: &shader,
                 entry_point: "fs_main",
                 targets: &[Some(wgpu_render_state.target_format.into())],
+                compilation_options: wgpu::PipelineCompilationOptions::default(),
             }),
             primitive: wgpu::PrimitiveState::default(),
             depth_stencil: None,