Refactor frame pacing from VT renderer into a separate class for sharing with other renderers

app/streaming/video/ffmpeg-renderers/pacer.cpp

@@ -0,0 +1,70 @@
+#include "pacer.h"
+
+#define FRAME_HISTORY_ENTRIES 8
+
+Pacer::Pacer() :
+    m_FrameQueueLock(0)
+{
+
+}
+
+Pacer::~Pacer()
+{
+    drain();
+}
+
+AVFrame* Pacer::getFrameAtVsync()
+{
+    SDL_AtomicLock(&m_FrameQueueLock);
+
+    int frameDropTarget;
+
+    // If the queue length history entries are large, be strict
+    // about dropping excess frames.
+    frameDropTarget = 1;
+    for (int i = 0; i < m_FrameQueueHistory.count(); i++) {
+        if (m_FrameQueueHistory[i] <= 1) {
+            // Be lenient as long as the queue length
+            // resolves before the end of frame history
+            frameDropTarget = 3;
+        }
+    }
+
+    if (m_FrameQueueHistory.count() == FRAME_HISTORY_ENTRIES) {
+        m_FrameQueueHistory.dequeue();
+    }
+
+    m_FrameQueueHistory.enqueue(m_FrameQueue.count());
+
+    // Catch up if we're several frames ahead
+    while (m_FrameQueue.count() > frameDropTarget) {
+        AVFrame* frame = m_FrameQueue.dequeue();
+        av_frame_free(&frame);
+    }
+
+    if (m_FrameQueue.isEmpty()) {
+        SDL_AtomicUnlock(&m_FrameQueueLock);
+        return nullptr;
+    }
+
+    // Grab the first frame
+    AVFrame* frame = m_FrameQueue.dequeue();
+    SDL_AtomicUnlock(&m_FrameQueueLock);
+
+    return frame;
+}
+
+void Pacer::submitFrame(AVFrame* frame)
+{
+    SDL_AtomicLock(&m_FrameQueueLock);
+    m_FrameQueue.enqueue(frame);
+    SDL_AtomicUnlock(&m_FrameQueueLock);
+}
+
+void Pacer::drain()
+{
+    while (!m_FrameQueue.isEmpty()) {
+        AVFrame* frame = m_FrameQueue.dequeue();
+        av_frame_free(&frame);
+    }
+}