linux-pipewire: Defer screencast portal init until source is shown

[avafloww]

Description

The previous behaviour would attempt screencast portal initialization even when the source is not shown, which can cause a flood of pop-ups, each indistinguishable from each other, on startup.

This commit defers portal init until the first time the source is shown, so the user isn't flooded with pop-ups when starting OBS.

Motivation and Context

This change addresses the ideas.obsproject.com post found here: https://ideas.obsproject.com/posts/2318/do-not-open-screen-sharing-portals-on-startup

On KDE Wayland, all screencast portal requests open as a separate window, with no further information on which source the request belongs to. In setups with many capture sources across multiple scenes, this can easily result in 10-20+ picker windows all opening at startup, with no way to differentiate them.

How Has This Been Tested?

I built the OBS Flatpak with my code changes and tested it.
The screencast portal opens correctly when sources are shown, and the source shows correctly once a window is confirmed via the portal.

Types of changes

• Tweak (non-breaking change to improve existing functionality)

Checklist:

• My code has been run through clang-format.
• I have read the contributing document.
• My code is not on the master branch.
• The code has been tested.
• All commit messages are properly formatted and commits squashed where appropriate.
• I have included updates to all appropriate documentation.