Add API for setting visibility of crop margins, render crop margins as boxes, fix transparency rendering

[seanzhang15]

This PR:

• adds a command to selectively visualize crop container margins
• renders crop margins as colored boxes instead of line meshes to better visualize the remaining volume
• adds labels to indicate the type of crop margin.
• switches to the proper logic for rendering transparency after tests have shown no difference in performance.

Crop-margin visualization:

[cielavenir]

It causes meshes to show the wireframes, which makes harder to view the scene.

Please make mesh behavior as before.

[cielavenir]

It is like this in production branch:

[cielavenir]

(I will restart the performance test after that issue is corrected)

[seanzhang15]

New image:

The outline is still displayed, but the ones in the back are properly covered by the robot, which is semi-transparent here.

cc @braineo

[cielavenir]

now it would be ok

[braineo]

transparent box inside another transparent box

[rdiankov]

thanks~