Add support for opaque 2D rendering

[djeedai]

Describe the solution you'd like

Bevy now supports separate opaque and transparent 2D render phases. For opaque 2D particles, we should use the opaque pass (and enable depth write).

Describe why you want that solution. Is this related to a problem?

• It's cleaner.
• It allows using depth write by default, which works around the lack of sorting.

Describe alternatives you've considered

Leave as is. This works now, just not optimal.

Additional context

There's no particle sorting, so the default rendering in the transparent phase with depth write disabled produces flickering particles.