Borrowed handle to the main viewport, including display/work area bounds.
Currently represents the Platform Window created by the application which is hosting our Dear ImGui windows.
In 'docking' branch with multi-viewport enabled, we extend this concept to have multiple active viewports.
In the future we will extend this concept further to also represent Platform Monitor and support a "no main platform window" operation mode.
return primary/default viewport. This can never be null.
Get the main viewport.