Allow color picking from reference images over the interface

[Cub0use]

Detailed description of your suggestion

In a paint tab for a model or in the image editor, we can only color pick from reference images inside the viewport/canvas

All the hashed areas in red are inaccessible to color picking

I can see why you'd not want to let that be possible for the modelling interface, but it would be nice to have it around the texturing space of the image editor where there are no buttons