[Viewer] Preserve gesture transformation matrix on slide/backend change

Change-Id: I417fb9b6d6b7c95884dff6871731883fe3736437
Reviewed-on: https://skia-review.googlesource.com/91460
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
3 files changed