Add UniqueHandle non-const accessors, ensure all by reference. (#123)

* Add UniqueHandle non-const accessors, ensure all by reference.

- Add `Type * operator->()`
- Fix `Type get() const` to `const Type & get() const`
- Add `Type & get()`

* Add support for structure pointer chains was missing some generator code.

* Update vulkan.hpp with changes.

VulkanHppGenerator.cpp

@@ -381,8 +381,8 @@ const std::string structureChainHeader = R"(
 
     StructureChain& operator=(StructureChain const &rhs)
     {
-      linkAndCopy(rhs);
-      return this;
+      linkAndCopy<StructureElements...>(rhs);
+      return *this;
     }
 
     template<typename ClassType> ClassType& get() { return static_cast<ClassType&>(*this);}
@@ -607,13 +607,28 @@ const std::string uniqueHandleHeader = R"(
     {
       return &m_value;
     }
-
+    
+    Type * operator->()
+    {
+      return &m_value;
+    }
+    
     Type const& operator*() const
     {
       return m_value;
     }
 
-    Type get() const
+    Type & operator*()
+    {
+      return m_value;
+    }
+
+    const Type & get() const
+    {
+      return m_value;
+    }
+    
+    Type & get()
     {
       return m_value;
     }

vulkan/vulkan.hpp

@@ -361,13 +361,28 @@ namespace vk
     {
       return &m_value;
     }
-
+    
+    Type * operator->()
+    {
+      return &m_value;
+    }
+    
     Type const& operator*() const
     {
       return m_value;
     }
 
-    Type get() const
+    Type & operator*()
+    {
+      return m_value;
+    }
+
+    const Type & get() const
+    {
+      return m_value;
+    }
+    
+    Type & get()
     {
       return m_value;
     }