viewer now takes --mskps <dir> and will have a slide per mskp and
overview slide (just like --skps).
Player uses offscreen surfaces to draw offscreen layers, allows
random access to mskp frames.
slide just plays mskp at fixed frame rate (for now).
Bug: skia:11900
Change-Id: I66104ffe88f5df721a1a835570acc3e4c23c3f07
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/400537
Commit-Queue: Brian Salomon <bsalomon@google.com>
Reviewed-by: Jim Van Verth <jvanverth@google.com>