Add SkSL editing to Viewer in Vulkan backend

Change-Id: Ic328dc9f16a1f1aa27306321f5a53b587c14fa21
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/210221
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Osman <brianosman@google.com>
8 files changed