Reland "FPS and frame control for MSKP slide."

This is a reland of 3b7587814de8f4b3c4970603ac4cd8565adb433c

Use SkTPin instead of std::clamp (C++17 library feature).

Original change's description:
> FPS and frame control for MSKP slide.
>
> Bug: skia:11900
> Change-Id: Ib4d8da6a86da7966e613de2d7cfd61ff545b296a
> Reviewed-on: https://skia-review.googlesource.com/c/skia/+/400676
> Commit-Queue: Brian Salomon <bsalomon@google.com>
> Reviewed-by: Jim Van Verth <jvanverth@google.com>

Bug: skia:11900
Change-Id: I29d3f6a717bbdd4e1fee3322e98d33e51a28f264
Cq-Include-Trybots: luci.skia.skia.primary:Housekeeper-PerCommit-CreateDockerImage_Skia_Release
Reviewed-on: https://skia-review.googlesource.com/c/skia/+/402917
Reviewed-by: Brian Salomon <bsalomon@google.com>
Commit-Queue: Brian Salomon <bsalomon@google.com>
diff --git a/tools/viewer/MSKPSlide.h b/tools/viewer/MSKPSlide.h
index 41b3c9c..bb9fa3e 100644
--- a/tools/viewer/MSKPSlide.h
+++ b/tools/viewer/MSKPSlide.h
@@ -28,10 +28,12 @@
 
 private:
     std::unique_ptr<SkStreamSeekable> fStream;
-    std::unique_ptr<MSKPPlayer> fPlayer;
-    int fFrame = 0;
-    int fFPS = 15;  // TODO: make this adjustable. This happens to work well for calendar.mskp
-    double fLastFrameTime = 0;
+    std::unique_ptr<MSKPPlayer>       fPlayer;
+
+    int    fFrame         = 0;
+    int    fFPS           = 15;
+    bool   fPaused        = false;
+    double fLastFrameTime = -1;
 
     using INHERITED = Slide;
 };