framework: fix pview crash when exit is called inside render_frame
framework: fix pview crash when exit is called inside render_frame

This can happen when a system event (eg. on macOS) triggers a terminate from within process_events.

This is a workaround for a common error; the proper fix is not to put PandaFramework in the global scope.

chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d,chandler14362/panda3d