[skottie] Show inval rects on mouse click in SkottieSlide

Also fix a SlideDir sizing bug.

TBR=

Change-Id: Iee61bb6665dc56ddd6fc062a686a9c0ef87dea92
Reviewed-on: https://skia-review.googlesource.com/108902
Reviewed-by: Florin Malita <fmalita@chromium.org>
Commit-Queue: Florin Malita <fmalita@chromium.org>
diff --git a/tools/viewer/SlideDir.cpp b/tools/viewer/SlideDir.cpp
index da8685d..ba3bbba 100644
--- a/tools/viewer/SlideDir.cpp
+++ b/tools/viewer/SlideDir.cpp
@@ -337,8 +337,8 @@
 }
 
 SkISize SlideDir::getDimensions() const {
-    return  SkSize::Make(fWinSize.width(),
-                         fCellSize.height() * (fSlides.count() / fColumns)).toCeil();
+    return SkSize::Make(fWinSize.width(),
+                        fCellSize.height() * (1 + (fSlides.count() - 1) / fColumns)).toCeil();
 }
 
 void SlideDir::draw(SkCanvas* canvas) {
@@ -414,5 +414,5 @@
               row = static_cast<int>(y / fCellSize.height()),
               idx = row * fColumns + col;
 
-    return idx <= fRecs.count() ? &fRecs[idx] : nullptr;
+    return idx < fRecs.count() ? &fRecs[idx] : nullptr;
 }