Implement the new checks for readonly stencils

[JCapucho]

Checklist

• Run cargo clippy. (clippy is complaining about other parts of the codebase)
• Run RUSTFLAGS=--cfg=web_sys_unstable_apis cargo clippy --target wasm32-unknown-unknown if applicable.
• Add change to CHANGELOG.md. See simple instructions inside file.

Connections
None that I know of

Description
wgpu currently checks if the write_mask is 0 to determine wether a stencil is used as readonly or not. However Webgpu contains a more complex ruleset that also checks the cull mode and face operations to determine if the stencil is readonly or not.

This PR aims to align the wgpu rules with the webgpu specification.

Testing
This changes was tested by using this patch and checking that the validation passes

diff --git a/wgpu/examples/shadow/main.rs b/wgpu/examples/shadow/main.rs
index 3b1f10d4..e2487835 100644
--- a/wgpu/examples/shadow/main.rs
+++ b/wgpu/examples/shadow/main.rs
@@ -519,7 +519,12 @@ impl framework::Example for Example {
                     format: Self::SHADOW_FORMAT,
                     depth_write_enabled: true,
                     depth_compare: wgpu::CompareFunction::LessEqual,
-                    stencil: wgpu::StencilState::default(),
+                    stencil: wgpu::StencilState {
+                        front: wgpu::StencilFaceState::IGNORE,
+                        back: wgpu::StencilFaceState::IGNORE,
+                        read_mask: !0,
+                        write_mask: !0,
+                    },
                     bias: wgpu::DepthBiasState {
                         constant: 2, // corresponds to bilinear filtering
                         slope_scale: 2.0,

and running some more complex applications that had the same problem.

[teoxoy]

Relevant spec section (for reference)

[teoxoy]

LGTM, needs a changelog entry though.

[JCapucho]

LGTM, needs a changelog entry though.

@teoxoy would this be considered a change or a bug fix?

[teoxoy]

would this be considered a change or a bug fix?

Could be either, I guess it depends which way you look at it. I usually use "change" if nobody opened an issue for it.

[JCapucho]

@teoxoy I've added the changelog entry and also added a little comment referencing the section of the specification.