gg: add a working implementation for screen_size() on windows (#13237)

Author: changrui

vlib/gg/gg.c.v

@@ -12,6 +12,11 @@ import sokol.sgl
 import sokol.gfx
 import math
 
+$if windows {
+	#flag -lgdi32
+	#include "windows.h"
+}
+
 pub type TouchPoint = C.sapp_touchpoint
 
 pub struct Event {
@@ -300,11 +305,20 @@ pub fn high_dpi() bool {
 	return C.sapp_high_dpi()
 }
 
+// call Windows API to get screen size
+fn C.GetSystemMetrics(int) int
+
 pub fn screen_size() Size {
 	$if macos {
 		return C.gg_get_screen_size()
 	}
-	// TODO windows, linux, etc
+	$if windows {
+		return Size{
+			width: int(C.GetSystemMetrics(C.SM_CXSCREEN))
+			height: int(C.GetSystemMetrics(C.SM_CYSCREEN))
+		}
+	}
+	// TODO linux, etc
 	return Size{}
 }