Add optional transparency passthrough for sprite backend with bevy_picking (#16388)

# Objective

- Allow bevy_sprite_picking backend to pass through transparent sections
of the sprite.
- Fixes #14929

## Solution

- After sprite picking detects the cursor is within a sprites rect,
check the pixel at that location on the texture and check that it meets
an optional transparency cutoff. Change originally created for
mod_picking on bevy 0.14
(aevyrie/bevy_mod_picking#373)

## Testing

- Ran Sprite Picking example to check it was working both with
transparency enabled and disabled
- ModPicking version is currently in use in my own isometric game where
this has been an extremely noticeable issue

## Showcase

![Sprite Picking
Text](https://github.com/user-attachments/assets/76568c0d-c359-422b-942d-17c84d3d3009)

## Migration Guide

Sprite picking now ignores transparent regions (with an alpha value less
than or equal to 0.1). To configure this, modify the
`SpriteBackendSettings` resource.

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Co-authored-by: andriyDev <[email protected]>

Author: vandie