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/news.dox

@@ -44,6 +44,16 @@ requesting a specific rendering backend when using
 contexts.
 
 
+@subsubsection features_34_init_allocator Support for custom memory allocator
+
+GLFW now supports plugging a custom memory allocator at initialization with @ref
+glfwInitAllocator.  The allocator is a struct of type @ref GLFWallocator with
+function pointers corresponding to the standard library functions `malloc`,
+`realloc` and `free`.
+
+For more information see @ref init_allocator.
+
+
 @subsubsection features_34_win32_keymenu Support for keyboard access to Windows window menu
 
 GLFW now provides the
@@ -118,7 +128,18 @@ then GLFW will fail to initialize.
 @subsection symbols_34 New symbols in version 3.4
 
 @subsubsection functions_34 New functions in version 3.4
+
+ - @ref glfwInitAllocator
+
+
 @subsubsection types_34 New types in version 3.4
+
+ - @ref GLFWallocator
+ - @ref GLFWallocatefun
+ - @ref GLFWreallocatefun
+ - @ref GLFWdeallocatefun
+
+
 @subsubsection constants_34 New constants in version 3.4
 
  - @ref GLFW_POINTING_HAND_CURSOR