Handle timers and multiple interactors in ProcessEvents
This fixes vtk x11 ProcessEvents inconsistent behavior and allows those who write custom event loops to leverage timer support in VTK (JoystickCamera, etc)
- Simply move out the event loop code into
ProcessEvents, so that it works just like one iteration of StartEventLoop.
- Use ProcessEvents in StartEventLoop.
I've tested that this change does not break previously fixed Regression on multiple render windows with VTK9 on a Linux system (CentOS 8, X.Org version: 1.12.2)