Loop videos seamlessly at the last decoded frame (#939)

Looped videos wrapped at the container duration, which is often longer
than the last frame the decoder delivers. The picture froze for the
difference before restarting. Track the actual end of the video stream
and wrap there instead.

Vocaluxe/Lib/Video/Acinerella/CDecoder.cs

@@ -124,12 +124,13 @@ namespace Vocaluxe.Lib.Video.Acinerella
             if (Loop)
             {
                 time = _LoopTime + _LoopTimer.ElapsedMilliseconds / 1000f;
-                if (time >= Length)
+                var loopLength = _Thread.LoopLength;
+                if (time >= loopLength)
                 {
                     do
                     {
-                        time -= Length;
-                    } while (time >= Length);
+                        time -= loopLength;
+                    } while (time >= loopLength);
 
                     _LoopTime = time;
                     _LoopTimer.Restart();

Vocaluxe/Lib/Video/Acinerella/CDecoderThread.cs

@@ -62,6 +62,7 @@ namespace Vocaluxe.Lib.Video.Acinerella
         private bool _DropSeekEnabled = true; // Used to fallback to frame skipping if seek is failing once on this file
 
         public float Length { get; private set; }
+        public float LoopLength { get; private set; }
         public bool Loop { get; set; }
 
         public CDecoderThread()
@@ -81,6 +82,7 @@ namespace Vocaluxe.Lib.Video.Acinerella
 
                 var instance = (SACInstance)Marshal.PtrToStructure(_Instance, typeof(SACInstance));
                 Length = instance.Info.Duration / 1000f;
+                LoopLength = Length;
                 var ok = instance.Opened && Length > 0.001f;
                 _DropSeekEnabled = true;
                 if (ok)
@@ -300,7 +302,7 @@ namespace Vocaluxe.Lib.Video.Acinerella
                     }
 
                     //If we don't the Length might be inaccurate (e.g. last frame ends at 19.98 but Length=20)
-                    if (time >= Length - 2 * _FrameDuration)
+                    if (time >= LoopLength - 2 * _FrameDuration)
                     {
                         return;
                     }
@@ -432,6 +434,11 @@ namespace Vocaluxe.Lib.Video.Acinerella
             {
                 if (Loop)
                 {
+                    if (!dropFrame && _LastDecodedTime > 0)
+                    {
+                        LoopLength = Math.Min(Length, _LastDecodedTime + _FrameDuration);
+                    }
+
                     RequestTime = _LoopedRequestTime;
                     _Skip();
                 }