Implement Premultiplied Alpha for OIT (#22821)

# Objective

- Allow rendering objects with `AlphaMode::Premultiplied` and
`AlphaMode::Add` while using Order Independent Transparency.
- Fixes #20373.

## Solution

- Store Premultiplied color in the OIT buffer and premultiply in the
case of `AlphaMode::Blend`, this shouldn't have any downsides since
blending was done with premultiplied alpha anyway.
- It might be useful to actively break the `oit_draw` signature by
adding a `premultiply` bool to ensure that shader authors are aware of
the changed meaning of the `color` argument.

## Testing

- These changes have been tested by running the modified
`order_independent_transparency` example, an older version of these
changes has been in use in a project i work on.

---

## Showcase

Here the Red Sphere uses `AlphaMode::Blend`, the Blue Sphere
`AlphaMode::Add` and the Green Sphere `AlphaMode::Premultiplied`.

<img width="1270" height="708" alt="bevy-oit-example"
src="https://github.com/user-attachments/assets/282d7083-458f-4ecd-b098-20b3539de796"
/>

---------

Signed-off-by: Schmarni <marnistromer@gmail.com>
Co-authored-by: Carter Anderson <mcanders1@gmail.com>