Co-authored-by Christopher Crawford [email protected]

closes flutter/flutter#85237
partially addresses flutter/flutter#85239

• Add a SPIRV FragmentShader interface in painting.dart
• Create end to end pixel tests for ops supported by the SPIRV to SkSL transpiler
• The end to end test also test the c++ fragment shader implementation

Pre-launch Checklist

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