Suggest enabling the Viewport to be built on a user-specified monitor at launch time

**Is your feature request related to a problem? Please describe.**

Sometimes in a multi-monitor environment (say, Monitors 1 and 2), user is willing to prepare some frames of content or something he could show to his audience. In this case, Monitor 1 is for the user to prepare and control the content, while Monitor 2 is for the audience to watch. Even commonly, the user can only see the screen of Monitor 1, but cannot see the screen of Monitor 2.

Actually, the audience can be either some people or some image/video capture cameras.

It would be of convenience that **the user could specify which Viewport window to be shown on which monitor**, so that he can control everything by watching Monitor 1 only ( e.g., he no longer needs to drag the screen from Monitor 1 to Monitor 2 and then maximize/fullscreen it, especially when he cannot see Monitor 2 at all).

**Describe the solution you'd like**

Add some settings or options to enable viewport window to be built on a specified monitor at launch time.

Start by locating egui's native viewport creation and launch-time window configuration. Determine how a user-specified monitor can select the viewport's initial display, then verify that the viewport opens on the requested monitor in a multi-monitor setup.

Written by the indexing model from the issue text.