Fix mappings for gamepads present at init

Joysticks already connected when GLFW was initalized did not get gamepad
mappings applied to them.

Regression introduced by 74a8ba26c3.

This change was backported without taking into account that 3.3.x does
not have on-demand joystick init.

Fixes #1996

src/input.c

@@ -401,6 +401,7 @@ void _glfwInputJoystickHat(_GLFWjoystick* js, int hat, char value)
 //
 void _glfwInitGamepadMappings(void)
 {
+    int jid;
     size_t i;
     const size_t count = sizeof(_glfwDefaultMappings) / sizeof(char*);
     _glfw.mappings = calloc(count, sizeof(_GLFWmapping));
@@ -410,6 +411,13 @@ void _glfwInitGamepadMappings(void)
         if (parseMapping(&_glfw.mappings[_glfw.mappingCount], _glfwDefaultMappings[i]))
             _glfw.mappingCount++;
     }
+
+    for (jid = 0;  jid <= GLFW_JOYSTICK_LAST;  jid++)
+    {
+        _GLFWjoystick* js = _glfw.joysticks + jid;
+        if (js->present)
+            js->mapping = findValidMapping(js);
+    }
 }
 
 // Returns an available joystick object with arrays and name allocated