WinRT: fixed bug where Win10 GameBar, when shown + hidden, might not restore a cursor's hidden state
The repro steps were this:
1. run an sdl2 winrt/uwp app, on Win10, v10.0.10586.0 or higher
2. hide the cursor, via a call to SDL_ShowCursor(0)
3. make the Win10 game bar appear, by pressing the Windows + G hotkey
4. observe that the mouse cursor appears, in order to interact with the
game bar (this is expected behavior)
5. make the Win10 game bar disappear, either by pressing the Windows + G hotkey
again, or clicking somewhere in the app
EXPECTED RESULT: cursor disappears, as game bar disappears
ACTUAL RESULT: cursor didn't always disappear
14 files changed
