Implement TextureFormat::Stencil8 + add stencil example

[Dinnerbone]

Checklist

• Run cargo clippy.
• 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
Fixes #3196

Description
It's common for 2d applications to use the stencil buffer without depth buffers, but wgpu so far required you to allocate (at minimum) a Depth24PlusStencil8, which can be wasteful. Stencil8 will, on supported platforms, only contain the stencil buffer and nothing else.

It currently maps to:

Backend	Texture Type	Notes
Vulkan	Either S8_UINT, D24_UNORM_S8_UINT or D32_SFLOAT_S8_UINT	Left to right, first one supported
GLES	GL_STENCIL_INDEX8	It's a required format in ES3
Metal	MTLPixelFormat.stencil8	It appears to be required in Metal, there's no mention to it being feature gated.
DX12	DXGI_FORMAT_D24_UNORM_S8_UINT	Effectively same as Depth24PlusStencil8
DX11	DXGI_FORMAT_D24_UNORM_S8_UINT	Effectively same as Depth24PlusStencil8

Testing
In addition to adding the new format to the existing clear_texture_tests, I've added an example called stencil-triangles which shows how to use stencil testing in wgpu.

All tests* pass for me on Windows and Mac.
*except for shadow on Mac, and water on Windows. Those seem to be known bugs and unrelated to my code though.

I've also confirmed that a large project I contribute to, Ruffle, runs and passes all its visual tests using Stencil8 on DX12, Vulkan, and GLES. I'm not able to run it on Metal currently.

[codecov-commenter]

Codecov Report

Merging #3343 (826bc21) into master (aa46e82) will increase coverage by 0.32%.
The diff coverage is 90.62%.

@@            Coverage Diff             @@
##           master    #3343      +/-   ##
==========================================
+ Coverage   64.10%   64.42%   +0.32%     
==========================================
  Files          86       86              
  Lines       42591    42622      +31     
==========================================
+ Hits        27301    27461     +160     
+ Misses      15290    15161     -129     

Impacted Files	Coverage Δ
wgpu-core/src/command/transfer.rs	75.14% <ø> (ø)
wgpu-core/src/validation.rs	58.87% <ø> (ø)
wgpu-hal/src/gles/adapter.rs	83.72% <ø> (ø)
wgpu-hal/src/vulkan/mod.rs	56.37% <ø> (ø)
wgpu-info/src/main.rs	76.41% <ø> (ø)
wgpu-hal/src/vulkan/conv.rs	74.72% <40.00%> (+2.95%)	⬆️
wgpu-hal/src/auxil/dxgi/conv.rs	76.02% <100.00%> (+0.33%)	⬆️
wgpu-hal/src/gles/conv.rs	75.28% <100.00%> (+0.93%)	⬆️
wgpu-hal/src/lib.rs	26.92% <100.00%> (+0.70%)	⬆️
wgpu-hal/src/vulkan/adapter.rs	76.77% <100.00%> (+0.33%)	⬆️
... and 16 more

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[nical]

Nice!