Allow storage of pipelines in custom shaders for WASM target

[svents]

I tried to add a custom shader component to my app (a boardgame with a web client and a local one) and ran into an error similar to this one:

error[E0277]: (dyn FnOnce(std::result::Result<(), BufferAccessError>) + 'static) cannot be sent between threads safely
--> examples/custom_shader/src/scene.rs:141:27
|
141 | storage.store(Pipeline::new(
| ____________________-----^
| | |
| | required by a bound introduced by this call
142 | | device,
143 | | queue,
144 | | format,
145 | | viewport.physical_size(),
146 | | ));

The traits to fix this are already available. To test it, I extended the custom shader example. Rebasing the first commit out will cause the error above.