improvements to the image editor pixel value inspector

Two improvements to the pixel values shown when right clicking an area of the image in the image editor:

1. Make the values always fit on screen to avoid having to make the editor full screen to see all of the values.

2. option to keep visible last two clicked pixel values. This will allow easier colour matching between for example the values of a backplate (first click), and the values 3d content whose values are currently being adjusted (second click).