Allow wgpu backend to be configured from the environment

[agorgl]

Multiple backends for wgpu can be available in a system.
Allow force changing the default pick through an environment variable.

This should be useful for setting the backend BackendBit::GL that will allow us to access gfx-backend-gl backend of gfx.

[mockersf]

hello 👋

Currently configuration is mostly done through Resource. There is one for WgpuRenderer, WgpuOptions, why not use it?

then in your app you could do

App::build().add_resource(WgpuOptions {
  backend: "gl", // this is not yet implemented, and could be an enum rather than a string
  ..Default::default()
})

[agorgl]

Because its up to the user to choose in which backend to run, we do not want it a compile time option.
For example a setup that supports both vulkan and opengl and has a bad vulkan driver could use this to override bevy wgpu backend on ALL bevy based games.

[cart]

Some random assorted thoughts:

1. Being able to easily override the backend in a built app is definitely useful (mainly for developers)
2. I do think backend configuration should live in WgpuOptions (and that it should be an enum). We can override WgpuOptions values from env variables, but the data should be passed into the renderer via WgpuOptions. It should be a way for devs (or knowledgeable users) to request a single, specific backend they know they want to run.
3. Users shouldn't ever need to specify backends manually to make an app work. This should be handled by wgpu's backend selection logic, and if its not, we should work with them to make that happen. If users need to set environment variables to make a game work (or even select a "compatible" backend from a settings menu), we have already failed. Thats not something we can expect the average user to know / care about.
4. I think we can get the desired "gl/dx11 fallback behavior" by setting the "backend bit" to BackendBit::PRIMARY | BackendBit::SECONDARY instead of just PRIMARY. We should probably set that as the default asap.

For the enum, I'm thinking it should be something like:

struct WgpuOptions {
  backend: WgpuBackend,
}

enum WgpuBackend {
  Default, // BackendBit::Primary | BackendBit::Secondary
  Vulkan, // BackendBit::Vulkan
  Gl, // BackendBit::Gl
  ...
}

[agorgl]

Yep totally agreed. I'm making some PRs on gfx-rs/gfx-backend-gl and I think we are gonna be able to run the basic examples from bevy with OpenGL backend pretty soon

[agorgl]

Made the changes. Did not set the secondary flag to the backend bit, as these indicate in-progress backends from the wgpu-rs/gfx team.

[cart]

Looks good to me!