Fix possible division by zero

On sway (a window manager), SDL_WINDOWEVENT_EXPOSED and
SDL_WINDOWEVENT_SIZE_CHANGED might not be called before a mouse event is
triggered. As a consequence, the "content rectangle" might not be
initialized when the mouse event is processed, causing a division by
zero.

To avoid the problem, initialize the content rect immediately when the
window is shown.

Fixes #4115 <https://github.com/Genymobile/scrcpy/issues/4115>