Fix for instance segmentation bug (#131)

* Fix for instance segmentation bug

Removed check for object ID 255 to be considered an invalid ID, it shouldn't have been, the logic already supported that ID. Added tests to verify result.

* Updated changelog

com.unity.perception/CHANGELOG.md

 
 ### Fixed
 
+Fixed a bug in instance segmentation labeler that erroneously logged that object ID 255 was not supported
+
 ## [0.6.0-preview.1] - 2020-12-03
 
 ### Added

com.unity.perception/Runtime/GroundTruth/InstanceIdToColorMapping.cs

 
         static uint GetColorForId(uint id)
         {
-            if (id > maxId || id == 0 || id == k_InvalidPackedColor) return k_InvalidPackedColor;
+            if (id > maxId || id == 0) return k_InvalidPackedColor;
 
             if (id <= k_HslCount)
             {

com.unity.perception/Tests/Runtime/GroundTruthTests/InstanceIdToColorMappingTests.cs

         }
 
         [Test]
+        public void InstanceIdToColorMappingTests_GetCorrectValuesFor255()
+        {
+            var expectedColor = new Color(0, 0, 191, 254);
+
+            Assert.IsTrue(InstanceIdToColorMapping.TryGetColorFromInstanceId(255u, out var color));
+            Assert.AreEqual(color.r, expectedColor.r);
+            Assert.AreEqual(color.g, expectedColor.g);
+            Assert.AreEqual(color.b, expectedColor.b);
+            Assert.AreEqual(color.a, expectedColor.a);
+            Assert.IsTrue(InstanceIdToColorMapping.TryGetInstanceIdFromColor(color, out var id2));
+            Assert.AreEqual(255u, id2);
+
+            color = InstanceIdToColorMapping.GetColorFromInstanceId(255u);
+            Assert.AreEqual(color.r, expectedColor.r);
+            Assert.AreEqual(color.g, expectedColor.g);
+            Assert.AreEqual(color.b, expectedColor.b);
+            Assert.AreEqual(color.a, expectedColor.a);
+            id2 = InstanceIdToColorMapping.GetInstanceIdFromColor(color);
+            Assert.AreEqual(255u, id2);
+        }
+
+        [Test]
-        [TestCase(255u)]
         public void InstanceIdToColorMappingTests_GetBlackForId(uint id)
         {
             Assert.IsFalse(InstanceIdToColorMapping.TryGetColorFromInstanceId(id, out var color));