Events: ensure the global event context is always available

The Boost unit tests have been failing since I removed the GUI2 event context since the
code expects both that A: the topmost context is the global one and B: that you can't
use join() to join the global context. Removing the UI event context meant the test's
fake_display event_context became the only (and global) context, causing an assert.

src/events.cpp

@@ -186,8 +186,8 @@ context::~context()
 // a new event context is created when e.g. a modal dialog is opened, and then
 // closed when that dialog is closed. Each context contains a list of the handlers
 // in that context. The current context is the one on the top of the stack.
-// The global context must always be in the first position.
-std::deque<context> event_contexts;
+// The global context must always be in the first position, so we initialize it here.
+std::deque<context> event_contexts(1);
 
 std::vector<pump_monitor*> pump_monitors;
 

src/wesnoth.cpp

@@ -1112,9 +1112,6 @@ int main(int argc, char** argv)
 	sigaction(SIGCHLD, &terminate_handler, nullptr);
 #endif
 
-	// declare this here so that it will always be at the front of the event queue.
-	events::event_context global_context;
-
 	SDL_StartTextInput();
 
 	try {