Fix y-axis sign for XInput thumb sticks

This breaks strict compatibility but does it to make XInput y-axes
consistent with every other API and OS.

Fixes #1083.

src/win32_joystick.c

@@ -711,9 +711,9 @@ int _glfwPlatformPollJoystick(_GLFWjoystick* js, int mode)
             return GLFW_TRUE;
 
         _glfwInputJoystickAxis(js, 0, (xis.Gamepad.sThumbLX + 0.5f) / 32767.5f);
-        _glfwInputJoystickAxis(js, 1, (xis.Gamepad.sThumbLY + 0.5f) / 32767.5f);
+        _glfwInputJoystickAxis(js, 1, -(xis.Gamepad.sThumbLY + 0.5f) / 32767.5f);
         _glfwInputJoystickAxis(js, 2, (xis.Gamepad.sThumbRX + 0.5f) / 32767.5f);
-        _glfwInputJoystickAxis(js, 3, (xis.Gamepad.sThumbRY + 0.5f) / 32767.5f);
+        _glfwInputJoystickAxis(js, 3, -(xis.Gamepad.sThumbRY + 0.5f) / 32767.5f);
         _glfwInputJoystickAxis(js, 4, xis.Gamepad.bLeftTrigger / 127.5f - 1.f);
         _glfwInputJoystickAxis(js, 5, xis.Gamepad.bRightTrigger / 127.5f - 1.f);