Contact.Distance() didn't store several calculation results into structs due to missing ref

Sensor bodies didn't detect contacts correctly due to distance calculations not storing data in structs because of missing "ref".
... and Fixture DebuggerDisplay string was incorrect.

Author: thomasvt

src/box2dx/Box2D.NetStandard/Collision/Simplex.cs

@@ -71,7 +71,7 @@ internal void ReadCache(
 			SimplexVertex[] vertices = m_v;
 			for (var i = 0; i < m_count; ++i)
 			{
-				SimplexVertex v = vertices[i];
+				ref SimplexVertex v = ref vertices[i];
 				v.indexA = cache.indexA[i];
 				v.indexB = cache.indexB[i];
 				Vector2 wALocal = proxyA.GetVertex(v.indexA);
@@ -98,7 +98,7 @@ internal void ReadCache(
 			// If the cache is empty or invalid ...
 			if (m_count == 0)
 			{
-				SimplexVertex v = vertices[0];
+				ref SimplexVertex v = ref vertices[0];
 				v.indexA = 0;
 				v.indexB = 0;
 				Vector2 wALocal = proxyA.GetVertex(0);

src/box2dx/Box2D.NetStandard/Dynamics/Contacts/Contact.cs

@@ -499,7 +499,7 @@ internal static void Distance(out DistanceOutput output, SimplexCache cache, in
 				}
 
 				// Compute a tentative new simplex vertex using support points.
-				SimplexVertex vertex = vertices[simplex.m_count];
+				ref SimplexVertex vertex = ref vertices[simplex.m_count];
 				vertex.indexA = proxyA.GetSupport(Math.MulT(transformA.q, -d));
 				vertex.wA = Math.Mul(transformA, proxyA.GetVertex(vertex.indexA));
 				vertex.indexB = proxyB.GetSupport(Math.MulT(transformB.q, d));

src/box2dx/Box2D.NetStandard/Dynamics/Fixtures/Fixture.cs

@@ -44,7 +44,7 @@ namespace Box2D.NetStandard.Dynamics.Fixtures
 	///  Fixtures are created via Body.CreateFixture.
 	///  @warning you cannot reuse fixtures.
 	/// </summary>
-	[DebuggerDisplay("Fixture of {m_body.m_userData}")]
+	[DebuggerDisplay("Fixture of {m_body.UserData}")]
 	public class Fixture
 	{
 		internal Body m_body;