Include OS detection in Emscripten joystick GUID

This PR modifies the Emscripten joystick backend to detect the user's OS and store its ID in the GUID, because different OSes might need different mappings for the same controllers.
I'm not sure if different browsers on the same OS can also have different mappings, but if they can, browser detection can be added to the GUID too if needed.

This PR also makes the GUID use `SDL_HARDWARE_BUS_USB` instead of `SDL_HARDWARE_BUS_UNKNOWN`, similarly to how Android and MFI backends always use `SDL_HARDWARE_BUS_BLUETOOTH` and GameInput, XInput, and RawInput backends always use `SDL_HARDWARE_BUS_USB`.

src/joystick/emscripten/SDL_sysjoystick.c

@@ -100,11 +100,33 @@ static int SDL_IsEmscriptenJoystickXInput(int device_index)
     }, device_index);
 }
 
+static int SDL_GetEmscriptenOSID()
+{
+    return MAIN_THREAD_EM_ASM_INT({
+        const os = ([
+            'Android',
+            'Linux',
+            'iPhone',
+            'Macintosh',
+            'Windows',
+        ]);
+        const ua = navigator['userAgent'];
+        for (let i = 0; i < os.length; i++) {
+            if (ua['indexOf'](os[i]) >= 0) {
+                return i + 1;
+            }
+        }
+        return 0;
+    });
+}
+
 static EM_BOOL Emscripten_JoyStickConnected(int eventType, const EmscriptenGamepadEvent *gamepadEvent, void *userData)
 {
     SDL_joylist_item *item;
     int i;
+    Uint16 bus;
     Uint16 vendor, product;
+    Uint8 os_id;
     bool is_xinput;
 
     SDL_LockJoysticks();
@@ -121,6 +143,7 @@ static EM_BOOL Emscripten_JoyStickConnected(int eventType, const EmscriptenGamep
     SDL_zerop(item);
     item->index = gamepadEvent->index;
 
+    bus = SDL_HARDWARE_BUS_UNKNOWN;
     vendor = SDL_GetEmscriptenJoystickVendor(gamepadEvent->index);
     product = SDL_GetEmscriptenJoystickProduct(gamepadEvent->index);
     is_xinput = SDL_IsEmscriptenJoystickXInput(gamepadEvent->index);
@@ -130,6 +153,21 @@ static EM_BOOL Emscripten_JoyStickConnected(int eventType, const EmscriptenGamep
         vendor = USB_VENDOR_MICROSOFT;
         product = USB_PRODUCT_XBOX360_XUSB_CONTROLLER;
     }
+    
+    os_id = SDL_GetEmscriptenOSID();
+
+    if (os_id != 0) {
+        if (os_id == 1 || os_id == 3) { // Android or iOS (mobile)
+            bus = SDL_HARDWARE_BUS_BLUETOOTH;
+        } else { // Desktop
+            bus = SDL_HARDWARE_BUS_USB;
+        }
+    }
+
+    if (SDL_strcmp(gamepadEvent->mapping, "standard") == 0) {
+        // We should differentiate between devices that are mapped or unmapped by the browser.
+        os_id += 0x80;
+    }
 
     item->name = SDL_CreateJoystickName(vendor, product, NULL, gamepadEvent->id);
     if (!item->name) {
@@ -138,9 +176,10 @@ static EM_BOOL Emscripten_JoyStickConnected(int eventType, const EmscriptenGamep
     }
 
     if (vendor && product) {
-        item->guid = SDL_CreateJoystickGUID(SDL_HARDWARE_BUS_UNKNOWN, vendor, product, 0, NULL, item->name, 0, 0);
+        item->guid = SDL_CreateJoystickGUID(bus, vendor, product, 0, NULL, gamepadEvent->id, 0, os_id);
     } else {
         item->guid = SDL_CreateJoystickGUIDForName(item->name);
+        item->guid.data[15] = os_id;
     }
 
     if (is_xinput) {