Implements the `GHOST_GetPixelAtCursor` for the X11 backend, allowing
the user to sample any color displayed on the screen.
There are a few known issues/limitations at the moment:
- Blender has no control of the cursor outside of its window, so it is
not going to be the eyedropper icon.
- Ghost does not report click events from outside of the window, so the
user needs to press Enter instead.
Ref #111303