NULL checking the main_context does not help as we've just destroyed the
GMainContext and set that field to NULL, not to mention it's unnecessary.
Fixes a leak of display's GSource.
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.