[0010] Allow casting to uint64_t (#134)

* [0010] Allow casting to uint64_t

We have decided to allow casting to uint64_t. It will make create
more opportunities for developers to make mistakes, but they do not
have to use it. The implementation cost does not seem too high.

Fixes #93

* Remove cast to bool

As Greg suggested, the cast to bool is no longer needed if we can cast
to an int, so we removed it. This commit also adds the uint64_t cast to
the pseudo class definition for BufferPointers.

* Fix linked list example after rebaseing.

proposals/0010-vk-buffer-ref.md

@@ -86,6 +86,7 @@ class vk::BufferPointer {
     vk::BufferPointer& operator=(const vk::BufferPointer&);
     vk::BufferPointer(const uint64_t);
     S& Get() const;
+    operator uint64_t() const;
 }
 ```
 
@@ -105,10 +106,10 @@ This new type will have the following operations
     vk::BufferPointer<DstType, DstAlign> only if SrcType is a type derived from
     DstType. vk::reinterpret_pointer_cast<T, A> allows casting for all other
     BufferPointer types. For both casts, DstAlign <= SrcAlign must be true.
-*   A buffer pointer can be constructed from a uint64_t u using the constructor
+*   A buffer pointer can be constructed from a uint64_t using the constructor
     syntax vk::BufferPointer<T,A>(u).
-*   A buffer pointer can be cast to a bool. If so, it returns FALSE if the
-    pointer is null, TRUE otherwise.
+*   A buffer pointer can be cast to a uint64_t. The cast will return the 64-bit
+    address that the pointer points to.
 
 Note the operations that are not allowed:
 
@@ -120,8 +121,12 @@ Note the operations that are not allowed:
 *   The comparison operators == and != are not supported for buffer pointers.
 
 Most of these restrictions are there for safety. They minimize the possibility
-of getting an invalid pointer. If the Get() method is used on a null or invalid
-pointer, the behaviour is undefined.
+of getting an invalid pointer. If a buffer pointer is cast to and from a
+uint64_t, then it is the responsibility of the user to make sure that a valid
+pointer is generated, and that aliasing rules are followed.
+
+If the Get() method is used on a null or invalid pointer, the behaviour is
+undefined.
 
 When used as a member in a buffer, vk::BufferPointer can be used to pass
 physical buffer addresses into a shader, and address and access buffer space
@@ -191,7 +196,7 @@ float4 MainPs(void) : SV_Target0
 {
       block_p g_p(g_PushConstants.root);
       g_p = g_p.Get().next;
-      if (!(bool)g_p) // Null pointer test
+      if ((uint64_t)g_pi == 0) // Null pointer test
           return float4(0.0,0.0,0.0,0.0);
       return g_p.Get().x
 }