Fix partial screen updating

Source/diablo.cpp

@@ -338,6 +338,7 @@ static void run_game_loop(unsigned int uMsg)
 			break;
 		if (!nthread_has_500ms_passed()) {
 			ProcessInput();
+			force_redraw |= 1;
 			DrawAndBlit();
 			continue;
 		}

Source/scrollrt.cpp

@@ -1545,7 +1545,7 @@ void DrawAndBlit()
 		return;
 	}
 
-	if (SCREEN_WIDTH > PANEL_WIDTH || SCREEN_HEIGHT > VIEWPORT_HEIGHT + PANEL_HEIGHT || force_redraw == 255) {
+	if (SCREEN_WIDTH > PANEL_WIDTH || force_redraw == 255) {
 		drawhpflag = TRUE;
 		drawmanaflag = TRUE;
 		drawbtnflag = TRUE;
@@ -1553,7 +1553,7 @@ void DrawAndBlit()
 		ddsdesc = FALSE;
 		ctrlPan = TRUE;
 		hgt = SCREEN_HEIGHT;
-	} else {
+	} else if (force_redraw == 1) {
 		ddsdesc = TRUE;
 		ctrlPan = FALSE;
 		hgt = VIEWPORT_HEIGHT;