Require a VelloView marker for cameras rendering Vello graphics

[msvbg]

It is common for Bevy cameras to mark what graphics features they use using a marker component, such as bloom or SSR, to name a few built-in effects that do this. In my game, I have cameras that don't need any Vello content at all. If I've read the logic correctly, they still pay the tax of running Vello code.

bevy_vello's render_frame system is currently the most demanding system in my game, and this PR seems to push it down to second place, although it is quite difficult to measure precisely.

Note that this is a breaking change, since users now have to add the VelloView component to their camera or nothing will render.

[msvbg]

Turns out I was unnecessarily redrawing a bunch of static lines every frame. Fixing that solved my performance problems, but I think this PR is still a good change.

[simbleau]

I'm in the middle of a bevy 0.15 refactor and will add your changes after the 0.15 upgrade. This might sit for a minute, apologies.