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/codec/SkWebpCodec.cpp b/src/codec/SkWebpCodec.cpp
index fe8ea3b..e901957 100644
--- a/src/codec/SkWebpCodec.cpp
+++ b/src/codec/SkWebpCodec.cpp
@@ -143,7 +143,7 @@
             // sense to guess kBGRA which is likely closer to the final
             // output.  Otherwise, we might end up converting
             // BGRA->YUVA->BGRA.
-            // Fallthrough:
+            [[fallthrough]];
         case 2:
             // This is the lossless format (BGRA).
             if (hasAlpha) {