Fix OIT depth test

[IceSentry]

Objective

• The depth test was only using the final depth but it should be testing each fragments
• Fully transparent fragments should not be added to the list

Solution

• Test each fragment after sorting

Testing

before:

after:

[hexroll]

LGTM.
I also tested this with two scenes and it's working as expected \o/

[hexroll]

I suppose this could be something like:

const EPSILON: f32 = 4.88e-04;
if color.a < EPSILON { ... }

But tbh I'm not seeing that this is done when comparing color values anywhere in the codebase.

[IceSentry]

The idea behind that comment is more about having a cutoff for almost transparent pixels. Instead of fully transparent, the reason is that every fragment we add to the list costs us more memory. Right now it doesn't really matter because we have a fixed buffer size, but eventually I want to work on making it smaller.

[tychedelia]

LGTM, might be worth filing a ticket so that we don't lose track unless this is something you think you'll get to for work anyway.

[IceSentry]

All the various TODOs in the OIT code are very much for future me when I'll work on it. Some of the improvements I need for work but some are a bevy specific.