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/gm/dstreadshuffle.cpp b/gm/dstreadshuffle.cpp
index 9e6a7ad..1737009 100644
--- a/gm/dstreadshuffle.cpp
+++ b/gm/dstreadshuffle.cpp
@@ -100,6 +100,7 @@
                 SkFont      font(ToolUtils::create_portable_typeface(), 100);
                 font.setEmbolden(true);
                 canvas->drawString(text, 0.f, 100.f, font, *paint);
+                break;
             }
             default:
                 break;