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>
10 files changed