Redraw windows on top when any window changes

This commit restores redraws of entire windows, but only if a window below
has actually changed. It should fix most regressions from commit c6d8692
with a much smaller CPU usage cost.

The commit also includes a check for the situation where the title screen
changes all the time, which would restore the high CPU usage (that the
developer would be unlikely to notice immediately).

src/gui/widgets/window.cpp

@@ -65,6 +65,10 @@
 
 #include "utils/functional.hpp"
 
+#include <algorithm>
+#include <iterator>
+#include <stdexcept>
+
 namespace wfl { class function_symbol_table; }
 namespace gui2 { class button; }
 
@@ -719,9 +723,23 @@ void window::draw()
 	}
 
 	if (dirty_list_.empty()) {
+		consecutive_changed_frames_ = 0u;
 		return;
 	}
 
+	++consecutive_changed_frames_;
+	if(consecutive_changed_frames_ >= 100u && id_ == "title_screen") {
+		/* The title screen has changed in 100 consecutive frames, i.e. every
+		frame for two seconds. It looks like the screen is constantly changing
+		or at least marking widgets as dirty.
+
+		That's a severe problem. Every time the title screen changes, all
+		other GUI windows need to be fully redrawn, with huge CPU usage cost.
+		For that reason, this situation is a hard error. */
+		throw std::logic_error("The title screen is constantly changing, "
+			"which has a huge CPU usage cost. See the code comment.");
+	}
+
 	for(auto & item : dirty_list_)
 	{
 
@@ -812,6 +830,8 @@ void window::draw()
 
 	dirty_list_.clear();
 
+	redraw_windows_on_top();
+
 	std::vector<widget*> call_stack;
 	populate_dirty_list(*this, call_stack);
 	assert(dirty_list_.empty());
@@ -1203,6 +1223,20 @@ void window_swap_grid(grid* g,
 
 } // namespace
 
+void window::redraw_windows_on_top() const
+{
+	auto me = std::find(open_window_stack.begin(), open_window_stack.end(), this);
+	if(me == open_window_stack.end()) {
+		// Known to happen for tooltips.
+		return;
+	}
+
+	for(auto it = std::next(me); it != open_window_stack.end(); ++it) {
+		// Note that setting an entire window dirty like this is expensive.
+		(*it)->set_is_dirty(true);
+	}
+}
+
 void window::finalize(const std::shared_ptr<builder_grid>& content_grid)
 {
 	window_swap_grid(nullptr, &get_grid(), content_grid->build(), "_window_content_grid");

src/gui/widgets/window.hpp

@@ -684,6 +684,16 @@ class window : public panel, public cursor::setter
 	 */
 	std::vector<std::vector<widget*> > dirty_list_;
 
+	/**
+	 * In how many consecutive frames the window has changed. This is used to
+	 * detect the situation where the title screen changes in every frame,
+	 * forcing all other windows to redraw everything all the time.
+	 */
+	unsigned int consecutive_changed_frames_ = 0u;
+
+	/** Schedules windows on top of us (if any) to redraw. */
+	void redraw_windows_on_top() const;
+
 	/**
 	 * Finishes the initialization of the grid.
 	 *