support clamp() in expressions, fix issue in importer

Runtime/SnappersHeadDefinitionMath.cs

 {
 	public static class SnappersHeadDefinitionMath
 	{
+		public static float clamp(float value, float min, float max)
+		{
+			return Mathf.Clamp(value, min, max);
+		}
+
-			return Math.Min(a, b);
+			return Mathf.Min(a, b);
-			return Math.Max(a, b);
+			return Mathf.Max(a, b);
 		}
 
 		public static float hermite(float p0, float p1, float r0, float r1, float t)

Runtime/SnappersHeadImporter.cs

 			sb.AppendLine();
 			sb.AppendFormat("	public class {0}_{1}\n", csClassPrefix, "SnappersHead : SnappersHeadDefinition");
 			sb.AppendLine("	{");
-			sb.AppendLine("		public override InstanceData CreateInstanceData(Mesh sourceMesh, Transform sourceRig, Warning warnings)");
+			sb.AppendLine("		public override InstanceData CreateInstanceData(Mesh sourceMesh, Transform sourceRig, Warnings warnings)");
 			sb.AppendLine("		{");
 			sb.AppendLine("			return CreateInstanceData<SnappersControllers, SnappersBlendShapes>(sourceMesh, sourceRig, warnings);");
 			sb.AppendLine("		}");