This is a reland of be056f4f62
The Switch test has been restructured to dodge an iOS bug.
Original change's description:
> Add switch statement support to PipelineStage.
>
> This allows us to write SKSL_TEST_ES3 tests in SkSLTest and have them
> run properly. Previously, such a test would assert inside the pipeline-
> stage generator. In ES2 mode, we will rewrite switches as chained ifs,
> but in ES3 mode we will want to continue emitting them as-is (they will
> be faster than chained ifs on a modern GPU).
>
> `writeSwitchStatement` is adapted from GLSLCodeGenerator.
>
> Change-Id: I532ea5ed49869e7cdffced0cdcd0e353af8d4d79
> Bug: skia:12450
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/450478
> Commit-Queue: John Stiles <johnstiles@google.com>
> Commit-Queue: Brian Osman <brianosman@google.com>
> Auto-Submit: John Stiles <johnstiles@google.com>
> Reviewed-by: Brian Osman <brianosman@google.com>
Bug: skia:12450
Change-Id: I5102081c636ef09cd23f5bc894e6c96e92a4c121
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/450757
Reviewed-by: Ethan Nicholas <ethannicholas@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
ed2babaf94