Fix implicit fallthroughs throughout Skia.

This CL is not fully comprehensive; for instance, it does not contain
fixes for backends that don't compile on Mac. But it does resolve the
vast majority of cases that trigger -Wimplicit-fallthrough.

A few minor bugs were found and fixed, but none that were likely to
affect normal operation.

Change-Id: I43487602b0d56200ce8b42702e04f66390d82f60
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/295916
Reviewed-by: Mike Klein <mtklein@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
diff --git a/src/sksl/SkSLGLSLCodeGenerator.cpp b/src/sksl/SkSLGLSLCodeGenerator.cpp
index 6f64ae5..127145a 100644
--- a/src/sksl/SkSLGLSLCodeGenerator.cpp
+++ b/src/sksl/SkSLGLSLCodeGenerator.cpp
@@ -522,7 +522,7 @@
                     this->write("-dFdy");
                     nameWritten = true;
                 }
-                // fallthru
+                [[fallthrough]];
             case FunctionClass::kDFdx:
             case FunctionClass::kFwidth:
                 if (!fFoundDerivatives &&