Move internals to shared joystick objects

Preparation for gamecontrollerdb support and the gamepad API.

src/osmesa_window.c

@@ -269,26 +269,11 @@ int _glfwPlatformGetKeyScancode(int key)
     return -1;
 }
 
-int _glfwPlatformJoystickPresent(int joy)
+int _glfwPlatformPollJoystick(int jid, int mode)
 {
     return GLFW_FALSE;
 }
 
-const float* _glfwPlatformGetJoystickAxes(int joy, int* count)
-{
-    return NULL;
-}
-
-const unsigned char* _glfwPlatformGetJoystickButtons(int joy, int* count)
-{
-    return NULL;
-}
-
-const char* _glfwPlatformGetJoystickName(int joy)
-{
-    return NULL;
-}
-
 void _glfwPlatformGetRequiredInstanceExtensions(char** extensions)
 {
 }