Ensure there are enough hwframes for Pacer

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

@@ -19,7 +19,9 @@
 // that the sum of all queued frames between both pacing and rendering queues
 // must not exceed the number buffer pool size to avoid running the decoder
 // out of available decoding surfaces.
-#define MAX_QUEUED_FRAMES 4
+#define MAX_QUEUED_FRAMES 3
+static_assert(PACER_MAX_OUTSTANDING_FRAMES == MAX_QUEUED_FRAMES + 2,
+              "PACER_MAX_OUTSTANDING_FRAMES and MAX_QUEUED_FRAMES must agree");
 
 // We may be woken up slightly late so don't go all the way
 // up to the next V-sync since we may accidentally step into

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

@@ -7,6 +7,12 @@
 #include <QMutex>
 #include <QWaitCondition>
 
+// The maximum number of frames pacer will ever hold is:
+// - 3 frames in the pacing queue
+// - 1 frame removed from the render queue in the process of rendering
+// - 1 frame for deferred free
+#define PACER_MAX_OUTSTANDING_FRAMES (3 + 1 + 1)
+
 class IVsyncSource {
 public:
     virtual ~IVsyncSource() {}