Revert update to wgpu 0.20 => downgrade to wgpu 0.19.1 (#4559)

0.20 has a bunch of bugs that will be fixed by:

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

At Rerun, we don't want to wait for the wgpu 0.20.1 patch release before
we update egui, so we will temporarily downgrade to wgpu 0.19

After reverting I'll open a new PR that will update to 0.20 again, with
the intention of merging that once 0.20.1 is released.

crates/egui_demo_app/src/apps/custom3d_wgpu.rs

@@ -49,13 +49,11 @@ 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,