Add more SampleApp support to viewer.

Fixes loading a Sample as first slide.
Adds char input.
Adds --slide and --list options.

Change-Id: I34b66818e3673fcfdc649443e7d9dfb74b478062
Reviewed-on: https://skia-review.googlesource.com/8445
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Jim Van Verth <jvanverth@google.com>
8 files changed