feat: add VelloDiagnosticsPlugin

[simbleau]

PR adds a new VelloDiagnosticsPlugin which tracks the number of SVG, Text, Lottie, and Scene assets being rendered each frame.

To do this, backend implementation was modified:

• Extracting now optimizes away assets not intersecting with a camera's render layers. (previously done in Render; this had to be moved to Extract since Extract is the only schedule with both access to the main and render world, where we synchronize back the frame data. This should be an improvement)
• Rendering now occurs in 2 steps: Sorting assets, then Rendering.

As well, a new example has been added, which shows a diagnostic:

[RobertBrewitz]

The extract and render changes are nice, shame to hide in a feature PR, worth splitting into a separate PR for tracking/change log purposes?

bevy_diagnostics is included in bevy_render which is a dependency of bevy_text and bevy_ui in this project; But, is there a case for putting VelloDiagnosticsPlugin and the scene counting behind a feature flag?

[simbleau]

The extract and render changes are nice, shame to hide in a feature PR, worth splitting into a separate PR for tracking/change log purposes?

bevy_diagnostics is included in bevy_render which is a dependency of bevy_text and bevy_ui in this project; But, is there a case for putting VelloDiagnosticsPlugin and the scene counting behind a feature flag?

I'd be open to a PR to feature-gate the diagnostics plugin, but currently not interested in taking up that work.

I will split this up into 2 PRs though, just found a way that makes sense.