Fixed connecting and disconnecting real-joysticks closing virtual joysticks in Emscripten (thanks David!)

Emscripten was using its own, private integer in order to allocate
new SDL_JoystickIDs.  SDL keeps a similar integer for allocating
joystick-ids, one which is shared across multiple joystick backends.

SDL 2.0.13 introduces a new joystick-backend, a Virtual joystick
backend, which allows for software-driven joysticks, and which is
designed to sit alongside joystick-backends that provide access to
physical joysticks.

The Emscripten and the Virtual backends were, at times, getting
allocated the same SDL_JoystickIDs, if and when both backends were used
simultaneously.  This could happen if, for example, an application
was using a virtual joystick in order to drive a touch-screen
based joystick, while also supporting physical joysticks through the
Emscripten backend.

When two joysticks end up with the same SDL_JoystickID, conflicts
can occur.  For example, disconnecting a physical joystick with
the same SDL_JoystickID as a virtual one, can lead to the virtual
joystick being closed, inadvertently.

This fix makes the Emscripten backend use SDL's cross-joystick-backend
integer counter, which is shared among joystick backends, for allocating
new SDL_JoystickIDs, rather than a private, Emscripten-specific
counter.

Fixes https://github.com/libsdl-org/SDL/issues/3647

(cherry picked from commit 07cb7c10a15b95387431bcb3a1ae77cfd432707b)

src/joystick/emscripten/SDL_sysjoystick.c

@@ -37,7 +37,6 @@ static SDL_joylist_item *JoystickByIndex(int index);
 static SDL_joylist_item *SDL_joylist = NULL;
 static SDL_joylist_item *SDL_joylist_tail = NULL;
 static int numjoysticks = 0;
-static int instance_counter = 0;
 
 static EM_BOOL Emscripten_JoyStickConnected(int eventType, const EmscriptenGamepadEvent *gamepadEvent, void *userData)
 {
@@ -72,7 +71,7 @@ static EM_BOOL Emscripten_JoyStickConnected(int eventType, const EmscriptenGamep
 
     item->naxes = gamepadEvent->numAxes;
     item->nbuttons = gamepadEvent->numButtons;
-    item->device_instance = instance_counter++;
+    item->device_instance = SDL_GetNextJoystickInstanceID();
 
     item->timestamp = gamepadEvent->timestamp;
 
@@ -168,7 +167,6 @@ static void EMSCRIPTEN_JoystickQuit(void)
     SDL_joylist = SDL_joylist_tail = NULL;
 
     numjoysticks = 0;
-    instance_counter = 0;
 
     emscripten_set_gamepadconnected_callback(NULL, 0, NULL);
     emscripten_set_gamepaddisconnected_callback(NULL, 0, NULL);