Add: [Win32] Automatic zoom level suggestion for Win32 video driver.

The zoom level suggestion is based on the DPI scaling set in Windows.
We use 150% scaling as the threshold for 2X zoom and 300% scaling
as the threshold for 4X zoom.

src/video/win32_v.h

@@ -43,6 +43,8 @@ public:
 
 protected:
 	Dimension GetScreenSize() const override;
+
+	float GetDPIScale() override;
 };
 
 /** The factory for Windows' video driver. */