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/win32_joystick.c

@@ -199,11 +199,11 @@ static GLFWbool supportsXInput(const GUID* guid)
     if (GetRawInputDeviceList(NULL, &count, sizeof(RAWINPUTDEVICELIST)) != 0)
         return GLFW_FALSE;
 
-    ridl = calloc(count, sizeof(RAWINPUTDEVICELIST));
+    ridl = _glfw_calloc(count, sizeof(RAWINPUTDEVICELIST));
 
     if (GetRawInputDeviceList(ridl, &count, sizeof(RAWINPUTDEVICELIST)) == (UINT) -1)
     {
-        free(ridl);
+        _glfw_free(ridl);
         return GLFW_FALSE;
     }
 
@@ -248,7 +248,7 @@ static GLFWbool supportsXInput(const GUID* guid)
         }
     }
 
-    free(ridl);
+    _glfw_free(ridl);
     return result;
 }
 
@@ -262,7 +262,7 @@ static void closeJoystick(_GLFWjoystick* js)
         IDirectInputDevice8_Release(js->win32.device);
     }
 
-    free(js->win32.objects);
+    _glfw_free(js->win32.objects);
 
     _glfwFreeJoystick(js);
     _glfwInputJoystick(js, GLFW_DISCONNECTED);
@@ -416,8 +416,8 @@ static BOOL CALLBACK deviceCallback(const DIDEVICEINSTANCE* di, void* user)
 
     memset(&data, 0, sizeof(data));
     data.device = device;
-    data.objects = calloc(dc.dwAxes + (size_t) dc.dwButtons + dc.dwPOVs,
-                          sizeof(_GLFWjoyobjectWin32));
+    data.objects = _glfw_calloc(dc.dwAxes + (size_t) dc.dwButtons + dc.dwPOVs,
+                                sizeof(_GLFWjoyobjectWin32));
 
     if (FAILED(IDirectInputDevice8_EnumObjects(device,
                                                deviceObjectCallback,
@@ -428,7 +428,7 @@ static BOOL CALLBACK deviceCallback(const DIDEVICEINSTANCE* di, void* user)
                         "Win32: Failed to enumerate device objects");
 
         IDirectInputDevice8_Release(device);
-        free(data.objects);
+        _glfw_free(data.objects);
         return DIENUM_CONTINUE;
     }
 
@@ -445,7 +445,7 @@ static BOOL CALLBACK deviceCallback(const DIDEVICEINSTANCE* di, void* user)
                         "Win32: Failed to convert joystick name to UTF-8");
 
         IDirectInputDevice8_Release(device);
-        free(data.objects);
+        _glfw_free(data.objects);
         return DIENUM_STOP;
     }
 
@@ -473,7 +473,7 @@ static BOOL CALLBACK deviceCallback(const DIDEVICEINSTANCE* di, void* user)
     if (!js)
     {
         IDirectInputDevice8_Release(device);
-        free(data.objects);
+        _glfw_free(data.objects);
         return DIENUM_STOP;
     }