Posting any message to parent seems to be pointless. That might break
parent window.
Regardless of the posting, parent window can catch mouse event
and also any keyboard events will be handled by parent window by default.
It's almost a fork of glwindow_win32 implementation.
To build on Windows, Vulkan SDK (at https://vulkan.lunarg.com/sdk/home)
and VK_SDK_PATH environment are required. Note that VK_SDK_PATH environment
setting is a part of the SDK installation.