events: Make debug logging of the event queue a hint instead of an #ifdef.

This makes it easy to toggle it on when debugging a new platform (or just
getting more visibility into an app) without having to rebuild SDL.

include/SDL_hints.h

@@ -1078,6 +1078,29 @@ extern "C" {
 #define SDL_HINT_RENDER_BATCHING  "SDL_RENDER_BATCHING"
 
 
+/**
+ *  \brief  A variable controlling whether SDL logs all events pushed onto its internal queue.
+ *
+ *  This variable can be set to the following values:
+ *
+ *    "0"     - Don't log any events (default)
+ *    "1"     - Log all events except mouse and finger motion, which are pretty spammy.
+ *    "2"     - Log all events.
+ *
+ *  This is generally meant to be used to debug SDL itself, but can be useful
+ *  for application developers that need better visibility into what is going
+ *  on in the event queue. Logged events are sent through SDL_Log(), which
+ *  means by default they appear on stdout on most platforms or maybe
+ *  OutputDebugString() on Windows, and can be funneled by the app with
+ *  SDL_LogSetOutputFunction(), etc.
+ *
+ *  This hint can be toggled on and off at runtime, if you only need to log
+ *  events for a small subset of program execution.
+ */
+#define SDL_HINT_EVENT_LOGGING   "SDL_EVENT_LOGGING"
+
+
+
 /**
  *  \brief  An enumeration of hint priorities
  */