RenderOptions should distinguish overridden from transient values

[jacobtylerwalls]

This is where the renderOptions and some sort of a status marker are in conflict -- we are using "startNewSystem" to mean both "override layout to start new system" and also "in this render should I start a new system?"

Originally posted by @mscuthbert in #126 (comment)

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We have overriddenWidth but nothing similar for the other render options, so when for instance, systemWidthsAndBreaks() sets left, width, displayClef, etc., we have only true/false values at our disposal, or possibly undefined, not a "never" or "always" or "overridden 400" etc.