Feature request: Add "safe area" bounding box (configurable) within display, and a keyboard toggle

[dshadoff]

At the very least for debug reasons, there are times when every pixel which the machine is capable of rendering should be shown - even if traditional display would obscure them to varying degrees.

The implementation until now has been to create "visible portion" definitions, and simply not display anything which lies outside of that definition. This feature request is to make the entire normal hardware output to be visible, but to display a "bounding box" marking around the area defined to be visible. This way, it is more obvious what is clearly visible, partially obscured, and off-screen for a given viewing aperture.