Implement Presentation Timestamp Correlation

[cwfitzgerald]

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

One step of #2869.

Description

Adds a way to get the appropriate presentation timestamps and correlate it with a user timestamp.

Testing

Manually tested using example.

[codecov-commenter]

Codecov Report

Merging #3240 (2870021) into master (f14bee6) will decrease coverage by 0.79%.
The diff coverage is 10.25%.

@@            Coverage Diff             @@
##           master    #3240      +/-   ##
==========================================
- Coverage   65.66%   64.86%   -0.80%     
==========================================
  Files          82       83       +1     
  Lines       39490    39607     +117     
==========================================
- Hits        25930    25692     -238     
- Misses      13560    13915     +355     

Impacted Files	Coverage Δ
wgpu-core/src/instance.rs	65.10% <0.00%> (-0.92%)	⬇️
wgpu-hal/src/dx11/adapter.rs	0.00% <0.00%> (ø)
wgpu-hal/src/dx12/mod.rs	27.35% <ø> (ø)
wgpu-hal/src/empty.rs	0.00% <0.00%> (ø)
wgpu-hal/src/gles/adapter.rs	83.61% <0.00%> (-0.36%)	⬇️
wgpu-hal/src/lib.rs	26.21% <ø> (ø)
wgpu-hal/src/vulkan/adapter.rs	76.14% <0.00%> (-1.37%)	⬇️
wgpu-types/src/lib.rs	88.09% <0.00%> (-0.21%)	⬇️
wgpu/src/backend/direct.rs	56.24% <0.00%> (-0.26%)	⬇️
wgpu/src/lib.rs	51.98% <0.00%> (-0.12%)	⬇️
... and 7 more

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[jimblandy]

This looks correct, but it seems more complicated than necessary. See comments.

[jimblandy]

Looks good to me! One doc fix, and one question: