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.

src/internal.h

@@ -527,6 +527,7 @@ struct _GLFWmutex
 struct _GLFWlibrary
 {
     GLFWbool            initialized;
+    GLFWallocator       allocator;
 
     struct {
         _GLFWinitconfig init;
@@ -798,3 +799,7 @@ char* _glfw_strdup(const char* source);
 float _glfw_fminf(float a, float b);
 float _glfw_fmaxf(float a, float b);
 
+void* _glfw_calloc(size_t count, size_t size);
+void* _glfw_realloc(void* pointer, size_t size);
+void _glfw_free(void* pointer);
+