Call SetStretchBltMode(HDC, HALFTONE) before StretchBlt

In theory, this will allow higher quality scaling in GDI mode if
the graphics driver supports it...

It doesn't look like the drive used for VirtualBox does though, so
I can't tell if it does anything or not.

src/conio/win32gdi.c

@@ -330,8 +330,11 @@ gdi_handle_wm_paint(HWND hwnd)
 	pthread_mutex_lock(&off_lock);
 	if (st == CIOLIB_SCALING_INTERNAL)
 		BitBlt(winDC, xoff, yoff, dwidth, dheight, memDC, 0, 0, SRCCOPY);
-	else
+	else {
+		SetStretchBltMode(winDC, HALFTONE);
+		SetBrushOrgEx(winDC, 0, 0, NULL);
 		StretchBlt(winDC, xoff, yoff, dwidth, dheight, memDC, 0, 0, sw, sh, SRCCOPY);
+	}
 	// Clear around image
 	if (xoff > 0) {
 		BitBlt(winDC, 0, 0, xoff - 1, h, memDC, 0, 0, BLACKNESS);