Add GPUCanvasContext getConfiguration() method

[beaufortfrancois]

As discussed in #4828, this PR exposes a way for web apps to feature-detect whether WebGPU can do HDR canvas. It does so by exposing a new GPUCanvasContext getConfiguration() method that returns the context configuration which includes device, format, usage, viewFormats, colorSpace, toneMapping, and alphaMode members.

[github-actions]

Previews, as seen when this build job started (3271365):
WebGPU _{webgpu.idl | Explainer | Correspondence Reference}
WGSL _{grammar.js | wgsl.lalr.txt}

[toji]

LGTM, thanks for the updates!

[kainino0x]

LGTM, that's explained well!

[kainino0x]

tiny nits

[greggman]

           **Returns:** {{GPUCanvasConfiguration}} or `null`

Sounds like a foot gun. Most people are going to write

const isReallyHDR = context.getConfiguration().toneMapping.mode === 'extended';

and then have their app crash when getConfiguration() returns null.

Also, when does it return null?

[beaufortfrancois]

and then have their app crash when getConfiguration() returns null.

They should write context.getConfiguration()?.toneMapping?.mode === 'extended'; instead.

We could do the same as with getCurrentTexture() and throw an InvalidStateError if this.[[configuration]] is null but I'm not sure I prefer this.

Also, when does it return null?

It returns null before the app calls configure() and after the app calls unconfigure().

[greggman]

It returns null before the app calls configure() and after the app calls unconfigure().

Thanks. I see that now.

context.getConfiguration()?.toneMapping?.mode === 'extended'

They shouldn't have to write this given that it only returns null before configure and after unconfigure.

[beaufortfrancois]

context.getConfiguration()?.toneMapping?.mode === 'extended'

They shouldn't have to write this given that it only returns null before configure and after unconfigure.

An external app monitoring the canvas configuration would have to do that but you're right, the app in control of the canvas would know.

[juj]

1. I find it a bit odd that the getter for GPUCanvasContext configuration would document behavior of how HDR tone mapping should work. Wouldn't this documentation be better suited to exist in GPUCanvasContext.configure() where these fields are established?

2. Also, there is

**Returns:** {{GPUCanvasConfiguration}} or `null`

When is this function allowed to return null? Can an implementation return null as a "eh, we didn't care to implement this function. We consider our WebGPU implementation to be a "pure" rendering device implementation (in the same philosophy of [D3DCREATE_PUREDEVICE](https://learn.microsoft.com/en-us/windows/win32/direct3d9/d3dcreate)) - track this information yourself."

3. Is this function guaranteed to return the exact same fields that were fed to GPUCanvasContext.configure()? Or can there be differences? For example, in the sense of "I configure()d with X, and now I'm calling getConfiguration() to see if I actually got Y" behavior? Can an implementation do that?

[kainino0x]

1. There are notes in the definitions for both toneMapping and configure() pointing toward it. Fair point it's an odd spot for that note though.
2. It returns null if and only if the canvas is not configured.
3. There aren't any steps which modify the configuration passed into configure() so the only change should be that keys not known by the browser are dropped, or if the requested configuration is invalid then configure() will fail and getConfiguration() will return null.

per specs for getConfiguration(), configure()