Auto merge of #27126 - kunalmohan:gpu-mapped-range, r=kvark

Implement GPUBuffer.getMappedRange()

<!-- Please describe your changes on the following line: -->
The new fields added to `GPUBuffer` are-
1. `mapping`- This is the buffer data. This is not directly exposed to user and is refcounted to keep track of the number of `ArrayBuffer`s that point to the content in it.
2. `mapping_range`- The range of mapped portion of the GPUBuffer.
3. `mapped_ranges`- Ranges that the various `ArrayBuffer`s expose to the user. Used for validation of `getMappedRange()`
4. `js_buffers`- Actual `ArrayBuffer`s that expose the data to the user. They are detached on unmap.
5. `map_promise`- Promise that represents the pending mapping.

This PR also changes the order of the exit of threads. WebGL thread is responsible for sending `Exit` message to WebRender and therefore should be exited after WebGPU threads.

r?@kvark @jdm

---
<!-- Thank you for contributing to Servo! Please replace each `[ ]` by `[X]` when the step is complete, and replace `___` with appropriate data: -->
- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [ ] These changes fix #___ (GitHub issue number if applicable)

<!-- Either: -->
- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because ___

<!-- Also, please make sure that "Allow edits from maintainers" checkbox is checked, so that we can help you if you get stuck somewhere along the way.-->

<!-- Pull requests that do not address these steps are welcome, but they will require additional verification as part of the review process. -->

0 files changed, 0 insertions, 0 deletions