Fix Win32 custom cursor set for non-client area

Udating the cursor via glfwSetCursor incorrectly included the non-client
area of the window.

src/win32_window.c

@@ -1319,6 +1319,7 @@ void _glfwPlatformDestroyCursor(_GLFWcursor* cursor)
 
 void _glfwPlatformSetCursor(_GLFWwindow* window, _GLFWcursor* cursor)
 {
+    RECT area;
     POINT pos;
 
     if (_glfw.cursorWindow != window)
@@ -1333,6 +1334,13 @@ void _glfwPlatformSetCursor(_GLFWwindow* window, _GLFWcursor* cursor)
     if (WindowFromPoint(pos) != window->win32.handle)
         return;
 
+    GetClientRect(window->win32.handle, &area);
+    ClientToScreen(window->win32.handle, (POINT*) &area.left);
+    ClientToScreen(window->win32.handle, (POINT*) &area.right);
+
+    if (!PtInRect(&area, pos))
+        return;
+
     if (cursor)
         SetCursor(cursor->win32.handle);
     else