* Fix meson build script for Windows. Since the Vulkan dependency
object was declared by us in case of Windows, the dependency object
shouldn't be used for finding header
* Fix build error by including Windows specific header
gstvkdisplay.c(563): error C2065: 'VK_KHR_WIN32_SURFACE_EXTENSION_NAME': undeclared identifier
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.