OnDemandRendering broken wrt expose events

Qt 3D does not handle expose events when targeting a window directly (i.e. running with the default, asynchronous render aspect). This is wrong.

With the default rendering model this is masked away since getting unexposed and then exposed again (on Windows this can be triggered by minimizing and restoring the window) seemingly works due to re-rendering no matter what, even if the scene itself is static.

Enabling on demand mode shows the problem: with static, non-animating scenes the newly exposed window will be empty until something else (e.g. a resize or a change in the scene) triggers rendering.

As an example, edit simple-cpp not to animate and enable on demand rendering. Or, launch "q3dsviewer -w" which does enable OnDemand and features a static scene when no uip is given. Then minimize and restore (on Windows - the expose semantics differ platform by platform)