3DSMax exporter to avoid negative frame index for animation (#1019)

* guard negative frame index

* put a guard on animation key frame

Author: pandaGaume

3ds Max/Max2Babylon/Exporter/BabylonExporter.Animation.cs

@@ -303,9 +303,9 @@ private List<BabylonAnimationKey> ExportBabylonKeysFromGameController(IIGameCont
 #else
                 var gameKey = gameKeyTab[new IntPtr(indexKey)];
 #endif
-
                 var key = new BabylonAnimationKey()
                 {
+                    // ensure the frame if not negative.
                     frame = gameKey.T / Loader.Global.TicksPerFrame,
                     values = extractValueFunc(gameKey)
                 };

SharedProjects/BabylonExport.Entities/BabylonAnimationKey.cs

@@ -8,8 +8,11 @@ namespace BabylonExport.Entities
     [DataContract]
     public class BabylonAnimationKey : IComparable<BabylonAnimationKey>, ICloneable
     {
+        private float _f;
+
         [DataMember]
-        public float frame { get; set; }
+        // guard for negative value.
+        public float frame { get => _f; set => _f = value < 0 ? 0 : value; }
 
         [DataMember]
         public float[] values { get; set; }