Add workaround for switch() containing only a `default` case.

The Mac Radeon GLSL driver crashes when given a switch statement that
only contains a default case and returns a value. Adding a case works
around the crash, and doesn't affect the meaning of the switch.

Change-Id: Iabbd267e0e31e8df7d3b7e747a7204d50931d0be
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/450977
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: John Stiles <johnstiles@google.com>
Auto-Submit: John Stiles <johnstiles@google.com>
8 files changed