* Remove generic parameter in compat::Manager.
The code is specific to bind group layout ids and the generic parameter gets in the way.
* Add a test.
The test actually covers wgpu's configuration where deduplication does not require the indirection rather than the new code. I used it to debug the new code with the configuration hard-coded. It's tedious to add a test to cover dedpuplication of bind group layouts for users of wgpu_core to provide their IDs, we can rely on the CTS which has test for that.
* Implement bind group layout deduplication for all configurations
Currently wgpu-core implement bind group layout deduplication only when it creates its own resource IDs. In other words it works for wgpu but not in Firefox.
This PR bridges the gap by allowing an optional indirection in bind group layouts: each BGL may store an ID referring to its "deduplicated" BGL.
When referring to a BGL the rest of the code must make sure to follow the indirection. The exception is command buffer processing which is considered hot code and where we first validate against the provided BGL ID and only follow the indirection if the initial check failed.
The main pain point with this approach is the various places where wgpu-core manually updates reference counts: we have to be careful about following the indirection to track the right BGL.
* Avoid making decisions based on the size of some generic type.
