fix: removed WinitPlugin from headless_renderer example

[cxreiff]

Objective

The headless_renderer example is meant to showcase running bevy as a headless renderer, but if run without a display server (for example, over an SSH connection), a panic occurs in bevy_winit despite never creating a window:

bevy_winit-0.14.1/src/lib.rs:132:14:
winit-0.30.5/src/platform_impl/linux/mod.rs:
neither WAYLAND_DISPLAY nor WAYLAND_SOCKET nor DISPLAY is set.

This example should run successfully in situations without an available display server, as although the GPU is used for rendering, no window is ever created.

Solution

Disabling WinitPlugin, where the above panic occurs, allows the example to run in a fully headless environment.

Testing

• I tested this change in normal circumstances with a display server (on macOS Sequoia and Asahi Linux) and behavior was normal.
• I tested with no display server by connecting via SSH, and running the example (on Asahi Linux). Previously this panics, but with this change it runs normally.

Considerations

• One could argue that ultimately the user should not need to remove WinitPlugin, and instead bevy should only throw the above panic when the application first attempts to create a window.

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[cxreiff]

Related to this change: from my own testing, the WindowPlugin settings above my change (e.g. setting primary_window to None) are not required, as they are redundant with adding ScheduleRunnerPlugin, which replaces the bevy_winit app runner and prevents a window from being created. Are there non-obvious performance reasons that the primary_window should be manually set to None? Or can these lines be removed?

DefaultPlugins.set(WindowPlugin {
   primary_window: None,
   exit_condition: bevy::window::ExitCondition::DontExit,
   close_when_requested: false,
}),