GDK-Win32: Clean up HiDPI support and WGL a bit

GDK-Win32: Clean up HiDPI support and WGL a bit

Make _gdk_win32_display_get_monitor_scale_factor() less complex, by:

*  Drop the preceding underscore.

*  Dropping an unused parameter.

*  Using a GdkSurface instead of a HWND, as the HWND that we pass into
   this function might have been taken from a GdkSurface, which are now
   always created with CS_OWNDC.  This means if a GdkSurface was passed
   in, we ensure that we only acquire the DC from the HWND once, and do
   not attempt to call ReleaseDC() on it.

*  Store the HDC that we acquire from the GdkSurface's HWND into the
   surface, and use that as the HDC we need for our GdkGLContext.

*  Drop the gl_hwnd from GdkWin32Display, as that is really should be
   stored in the GdkSurface.

*  For functions that were updated, name GdkWin32Display variables as
   display_win32 and GdkSurface variables as surface, to unify things.

*  Stop calling ReleaseDC() on the HDC that we use for OpenGL, since
   they were acquired from HWND's created with CS_OWNDC.