Eyedropper tool

## What problem does this solve or what need does it fill?

In the current Bevy editor mocks, the color picker incudes an eyedropper tool. Currently there's no way to support this in Bevy.

## What solution would you like?

If we want to sample colors from outside the Bevy window, there are two problems that need to be solved:

* Capturing the pointer so that click events don't trigger other apps
* Getting the screen coordinates of the click event
* Sampling the pixel color at that point

These will require native OS code, as these features aren't provided through winit. These can go into the platform crate.

## What alternative(s) have you considered?

A less ambitious approach is to only support picking colors from inside the bevy window.

Start by reviewing the platform crate and the current winit input handling, focusing on pointer capture, screen-coordinate retrieval, and pixel sampling. Compare the native OS requirements across supported platforms and define whether the implementation should support outside-window sampling or only the Bevy window; done means the eyedropper can reliably select the intended color without triggering other applications.

Written by the indexing model from the issue text.