Fix calling chain in galleryslide functions.

The clang-tidy "bugprone parent virtual call" checker pointed out a
couple of places where galleryslide functions call directly up to
their grandparent function instead of their (in this case
non-existent) parent function.  Add new parent functions that do
nothing more than call up, and call those instead of directly calling
the grandparent.

https://clang.llvm.org/extra/clang-tidy/checks/bugprone-parent-virtual-call.html

mythtv/programs/mythfrontend/galleryslide.cpp

@@ -163,9 +163,7 @@ void SequentialAnimation::Start(bool forwards, float speed)
     m_current = forwards ? 0 : m_group.size() - 1;
 
     // Start group, then first child
-    // Parent function explicitly set to zero. Have to call
-    // grandparent directly to get work done.
-    AbstractAnimation::Start(forwards, speed);
+    GroupAnimation::Start(forwards, speed);
     m_group.at(m_current)->Start(m_forwards, m_speed);
 }
 
@@ -177,9 +175,7 @@ void SequentialAnimation::Start(bool forwards, float speed)
 void SequentialAnimation::SetSpeed(float speed)
 {
     // Set group speed for subsequent children
-    // Parent function explicitly set to zero. Have to call
-    // grandparent directly to get work done.
-    AbstractAnimation::SetSpeed(speed);
+    GroupAnimation::SetSpeed(speed);
 
     // Set active child
     if (!m_running || m_current < 0 || m_current >= m_group.size())
@@ -230,9 +226,7 @@ void ParallelAnimation::Start(bool forwards, float speed)
     m_finished = m_group.size();
 
     // Start group, then all children
-    // Parent function explicitly set to zero. Have to call
-    // grandparent directly to get work done.
-    AbstractAnimation::Start(forwards, speed);
+    GroupAnimation::Start(forwards, speed);
     foreach(AbstractAnimation *animation, m_group)
         animation->Start(m_forwards, m_speed);
 }
@@ -245,9 +239,7 @@ void ParallelAnimation::Start(bool forwards, float speed)
 void ParallelAnimation::SetSpeed(float speed)
 {
     // Set group speed, then all children
-    // Parent function explicitly set to zero. Have to call
-    // grandparent directly to get work done.
-    AbstractAnimation::SetSpeed(speed);
+    GroupAnimation::SetSpeed(speed);
     foreach(AbstractAnimation *animation, m_group)
         animation->SetSpeed(m_speed);
 }

mythtv/programs/mythfrontend/galleryslide.h

@@ -94,8 +94,10 @@ class GroupAnimation : public AbstractAnimation
     GroupAnimation() : AbstractAnimation()            {}
     virtual ~GroupAnimation()                         { GroupAnimation::Clear(); }
     void Pulse(int interval) override                     = 0; // AbstractAnimation
-    void Start(bool forwards, float speed = 1.0) override = 0; // AbstractAnimation
-    void SetSpeed(float speed) override                   = 0; // AbstractAnimation
+    void Start(bool forwards, float speed = 1.0) override      // AbstractAnimation
+        { AbstractAnimation::Start(forwards, speed); }
+    void SetSpeed(float speed) override                        // AbstractAnimation
+        { AbstractAnimation::SetSpeed(speed); }
     virtual void Add(AbstractAnimation *child);
     void Clear() override; // AbstractAnimation