Add pluggable heap allocator

This adds the glfwInitAllocator function for specifying a custom memory
allocator to use instead of the C runtime library.

The allocator is a struct of type GLFWallocator with fields
corresponding to malloc, realloc and free, while the internal API
corresponds to calloc, realloc and free.

Heap allocation calls are filtered before reaching the user-provided
functions, so deallocation of NULL and allocations of zero bytes are not
passed on, reallocating NULL is transformed into an allocation and
reallocating to size zero is transformed into deallocation.

The clearing of a new block to zero is performed by the internal
calloc-like function.

Closes #544.
Fixes #1628.
Closes #1947.

docs/intro.dox

@@ -33,6 +33,7 @@ successfully initialized, and only from the main thread.
  - @ref glfwGetError
  - @ref glfwSetErrorCallback
  - @ref glfwInitHint
+ - @ref glfwInitAllocator
  - @ref glfwInit
  - @ref glfwTerminate
 
@@ -143,6 +144,64 @@ Initialization hint              | Default value                   | Supported v
 @ref GLFW_X11_XCB_VULKAN_SURFACE | `GLFW_TRUE`                     | `GLFW_TRUE` or `GLFW_FALSE`
 
 
+@subsection init_allocator Custom heap memory allocator
+
+The heap memory allocator can be customized before initialization with @ref
+glfwInitAllocator.
+
+@code
+GLFWallocator allocator;
+allocator.allocate = my_malloc;
+allocator.reallocate = my_realloc;
+allocator.deallocate = my_free;
+allocator.user = NULL;
+
+glfwInitAllocator(&allocator);
+@endcode
+
+The allocator will be picked up at the beginning of initialization and will be
+used until GLFW has been fully terminated.  Any allocator set after
+initialization will be picked up only at the next initialization.
+
+The allocator will only be used for allocations that would have been made with
+the C standard library.  Memory allocations that must be made with platform
+specific APIs will still use those.
+
+The allocation function must have a signature matching @ref GLFWallocatefun.  It receives
+the desired size, in bytes, and the user pointer passed to @ref glfwInitAllocator and
+returns the address to the allocated memory block.
+
+@code
+void* my_malloc(size_t size, void* user)
+{
+    ...
+}
+@endcode
+
+The reallocation function must have a function signature matching @ref GLFWreallocatefun.
+It receives the memory block to be reallocated, the new desired size, in bytes, and the user
+pointer passed to @ref glfwInitAllocator and returns the address to the resized memory
+block.
+
+@code
+void* my_realloc(void* block, size_t size, void* user)
+{
+    ...
+}
+@endcode
+
+The deallocation function must have a function signature matching @ref GLFWdeallocatefun.
+It receives the memory block to be deallocated and the user pointer passed to @ref
+glfwInitAllocator.
+
+@code
+void my_free(void* block, void* user)
+{
+    ...
+}
+@endcode
+
+
 @subsection intro_init_terminate Terminating GLFW
 
 Before your application exits, you should terminate the GLFW library if it has