Add support for mouse input transparency

This adds the GLFW_MOUSE_PASSTHROUGH window hint and attribute for
controlling whether mouse input passes through the window to whatever
window is behind it.

Fixes #1236.
Closes #1568.

src/win32_window.c

@@ -1201,6 +1201,13 @@ static LRESULT CALLBACK windowProc(HWND hWnd, UINT uMsg,
             DragFinish(drop);
             return 0;
         }
+
+        case WM_NCHITTEST:
+        {
+            if (window->mousePassthrough)
+                return HTTRANSPARENT;
+            break;
+        }
     }
 
     return DefWindowProcW(hWnd, uMsg, wParam, lParam);
@@ -1854,6 +1861,11 @@ void _glfwPlatformSetWindowFloating(_GLFWwindow* window, GLFWbool enabled)
                  SWP_NOACTIVATE | SWP_NOMOVE | SWP_NOSIZE);
 }
 
+void _glfwPlatformSetWindowMousePassthrough(_GLFWwindow* window, GLFWbool enabled)
+{
+    window->mousePassthrough = enabled;
+}
+
 float _glfwPlatformGetWindowOpacity(_GLFWwindow* window)
 {
     BYTE alpha;