Enable Multi-Viewports on this lib

[zaafar]

it will allow users to create ImGuiWindows outside the SDL2 Window (in case overlay window is too small OR they have multiple monitors).

[zaafar]

This will yield a really bad user experience. For example, if you move an ImGui window to another monitor (by using the multi-viewports feature). That ImGui window will overlay whatever is opened on the other monitor e.g. your browser. Also, overlay (by definition) can not be alt-tabbed or minimized. So basically the user is stuck with an overlay window on their browser.