[Impeller] cleaned up semantics for RenderPipelineT and added docstrings (#52237)

I just tried to capture all thing things I found confusing about these
and clean them up. We were mixing up Pipeline and RenderPipelineT in our
generics arguments and RenderPipelineT wasn't clear since it was named
like a template parameter but was a runtime type.

Summary of edits:
-  RenderPipelineT -> RenderPipelineHandle
- Added docstrings for RenderPipelineHandle and Variants
- cleaned up generics parameters to ContentContext methods and Variants
- replaced references that called the variant's default pipeline handle
a "prototype" since it clashed with places that just call it the
"default".

## Pre-launch Checklist

- [x] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [x] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [x] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [x] I listed at least one issue that this PR fixes in the description
above.
- [x] I added new tests to check the change I am making or feature I am
adding, or the PR is [test-exempt]. See [testing the engine] for
instructions on writing and running engine tests.
- [x] I updated/added relevant documentation (doc comments with `///`).
- [x] I signed the [CLA].
- [x] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

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3 files changed
tree: ee60628a450b6937d7ae8b2efe2f1b3491f51bb0
  1. .github/
  2. assets/
  3. benchmarking/
  4. bin/
  5. build/
  6. build_overrides/
  7. ci/
  8. common/
  9. display_list/
  10. docs/
  11. examples/
  12. flow/
  13. flutter_frontend_server/
  14. flutter_vma/
  15. fml/
  16. impeller/
  17. lib/
  18. runtime/
  19. shell/
  20. skia/
  21. sky/
  22. testing/
  23. third_party/
  24. tools/
  25. vulkan/
  26. wasm/
  27. web_sdk/
  28. .ci.yaml
  29. .clang-format
  30. .clang-tidy
  31. .clangd
  32. .engine-release.version
  33. .gitattributes
  34. .gitignore
  35. .pylintrc
  36. .style.yapf
  37. analysis_options.yaml
  38. AUTHORS
  39. BUILD.gn
  40. CODEOWNERS
  41. CONTRIBUTING.md
  42. DEPS
  43. Doxyfile
  44. LICENSE
  45. README.md
README.md

Flutter Engine

Flutter CI Status OpenSSF Scorecard SLSA 1

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