Show dead/non-linear zones in monitor

[nathanmacinnes]

Source/filter types

• Face Tracker (source)
• Face Tracker (filter)
• Face Tracker PTZ (a filter to control PTZ camera)
• Face Tracker Monitor

Is your feature request related to a problem? Please describe.

It's tricky to visualise where the face is in relation to the dead zone in monitor/debug.

Describe the solution you'd like

A square in a different colour showing the dead zone, and perhaps a larger square around it showing the non-linear zone.

Describe alternatives you've considered

Two pairs of lines (one pair horizontal and one vertical), but this would probably be more confusing than a square.

Additional context