Make it clear in UI, when graphics features are not available

[hannobraun]

As of #902, the renderer checks which graphics features are available, and only attempts to do operations that actually will work. Right now, this only affects line drawing (displaying the model mesh, and debug output).

However, this information is not available in the UI. I think ideally, the UI elements for enabling anything that can't be provided by the graphics hardware should be disabled (and displayed greyed-out, or however egui does it), and there should be information explaining why the UI element is inactive. At least in a tooltip, and possibly even by displaying some text next to it.

It would probably be best to add methods like is_line_drawing_available(&self) -> bool) to Renderer, so code outside of Renderer doesn't have to deal with wgpu::Features or other wgpu-specific stuff (just a suggestion; it could be named or implemented differently).

Labeling https://github.com/hannobraun/Fornjot/labels/good%20first%20issue, as this should be a fairly isolated change (limited to rendering/GUI) and https://github.com/hannobraun/Fornjot/labels/type%3A%20bug, as the UI elements would just not work for affected users.