[Pharo-project] what's the standard way of finding out from Smalltalk if one is running headless?

[Eliot Miranda]

I thought to test things like Display extent & DisplayScreen
actualScreenDepth but these either fall back to values in the pre-headless
image or don't fail and answer the actual display depth, even though the VM
is running headless (at least on Mac they do).
-- 
best,
Eliot