emscripten: Introduce SDL_HINT_EMSCRIPTEN_ASYNCIFY

See https://github.com/emscripten-core/emscripten/issues/10746

and

https://github.com/emscripten-ports/SDL2/pull/112

Fixes Bugzilla #4997.

include/SDL_hints.h

@@ -1042,6 +1042,21 @@ extern "C" {
  */
 #define SDL_HINT_EMSCRIPTEN_KEYBOARD_ELEMENT   "SDL_EMSCRIPTEN_KEYBOARD_ELEMENT"
 
+/**
+ *  \brief Disable giving back control to the browser automatically
+ *  when running with asyncify
+ *
+ * With -s ASYNCIFY, SDL2 calls emscripten_sleep during operations
+ * such as refreshing the screen or polling events.
+ *
+ * This hint only applies to the emscripten platform
+ *
+ * The variable can be set to the following values:
+ *    "0"       - Disable emscripten_sleep calls (if you give back browser control manually or use asyncify for other purposes)
+ *    "1"       - Enable emscripten_sleep calls (the default)
+ */
+#define SDL_HINT_EMSCRIPTEN_ASYNCIFY   "SDL_EMSCRIPTEN_ASYNCIFY"
+
 /**
  *  \brief Tell SDL not to catch the SIGINT or SIGTERM signals.
  *