Improve correctness of hotkeys at low framerates.

SDL queues up all the input events received in a frame. When the hotkey
system saw a key up/down event, it immediately updated its
HotkeyIsPressed state and then pushed a hotkey event onto the end of the
queue.

If the initial queue was e.g. [key-down shift, key-press Z, key-up
shift], the hotkey event triggered by Z would be processed after the
key-up shift had updated the HotkeyIsPressed state, so the handler of
the Z hotkey would not think the shift hotkey was pressed.

If the initial queue was e.g. [key-press Z, mouse-click], the hotkey
triggered by Z would be processed after the mouse-click event, so it
could apply to the wrong building selection.

Fix by pushing the hotkey events onto a special queue that gets
processed before any subsequent SDL input events.

Also update the HotkeyIsPressed status when the HOTKEYDOWN/HOTKEYUP
events are processed, not when they are generated, to guarantee they are
consistent with the DOWN/UP events.

Fixes #1869.

This was SVN commit r14057.

source/main.cpp

@@ -180,7 +180,7 @@ static void PumpEvents()
 	PROFILE3("dispatch events");
 
 	SDL_Event_ ev;
-	while (SDL_PollEvent(&ev.ev))
+	while (in_poll_event(&ev))
 	{
 		PROFILE2("event");
 		if (g_GUI)