Viewer: Fixes for shader editing in ANGLE

- Include default precision in viewer's highlight shader when needed
- Flush cache when switching backends. Fixes issues between (eg) GL
  and ANGLE where shader caps are different.

Change-Id: I80bc9fb56fdab49fdbe2e858db7398a5471048c6
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/212194
Commit-Queue: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
Auto-Submit: Brian Osman <brianosman@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>
1 file changed