Win: Use a zeroed out MARGINS struct when not using a "composited" or dark mode title bar Window. When not composited or using the dark mode titlebar (drawing in the titlebar) with NCCALCSIZE we should reset the client area to default by using a zeroed MARGINS. This fixes some display issues introduced by some dark mode titlebar and always composite changes earlier this year. Also added some debug code to help diagnose dark mode titlebar issues, enabed with DEBUG=Y