patch 8.1.2301: MS-Windows GUI: drawing error when background color changes

Problem:    MS-Windows GUI: drawing error when background color changes.
Solution:   Implement gui_mch_new_colors(). (Simon Sadler)
diff --git a/src/gui_w32.c b/src/gui_w32.c
index 02d2e09..3dbbc5e 100644
--- a/src/gui_w32.c
+++ b/src/gui_w32.c
@@ -325,7 +325,7 @@
 #endif
 HWND			s_hwnd = NULL;
 static HDC		s_hdc = NULL;
-static HBRUSH	s_brush = NULL;
+static HBRUSH		s_brush = NULL;
 
 #ifdef FEAT_TOOLBAR
 static HWND		s_toolbarhwnd = NULL;
@@ -1282,7 +1282,18 @@
     void
 gui_mch_new_colors(void)
 {
-    /* nothing to do? */
+    HBRUSH prevBrush;
+
+    s_brush = CreateSolidBrush(gui.back_pixel);
+#ifdef SetClassLongPtr
+    prevBrush = (HBRUSH)SetClassLongPtr(
+				s_hwnd, GCLP_HBRBACKGROUND, (LONG_PTR)s_brush);
+#else
+    prevBrush = (HBRUSH)SetClassLong(
+				   s_hwnd, GCL_HBRBACKGROUND, (long_u)s_brush);
+#endif
+    InvalidateRect(s_hwnd, NULL, TRUE);
+    DeleteObject(prevBrush);
 }
 
 /*