Fix Viewer slide load on backend change - take 2

Split the slide switching logic (including load/unload) into
setCurrentSlide(), while keeping setupCurrentSlide() to deal with config
only.

Change-Id: I5bd2363ffd401c1b756217f845d4dbd16d6be5d6
Reviewed-on: https://skia-review.googlesource.com/94864
Reviewed-by: Brian Osman <brianosman@google.com>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/tools/viewer/Viewer.h b/tools/viewer/Viewer.h
index 7d4980c..5067952 100644
--- a/tools/viewer/Viewer.h
+++ b/tools/viewer/Viewer.h
@@ -49,9 +49,10 @@
     void updateTitle();
     void setBackend(sk_app::Window::BackendType);
     void setColorMode(ColorMode);
-    void setStartupSlide();
-    void setupCurrentSlide(int previousSlide);
-    void listNames();
+    int startupSlide() const;
+    void setCurrentSlide(int);
+    void setupCurrentSlide();
+    void listNames() const;
 
     void updateUIState();