Interface StyleOverride

Partial-update patch for the renderer's global style override — the nullable mirror of OverridableStyle. Each field is independent:

• undefined — leave this property's override untouched.
• null — clear the override for this property (per-segment value wins).
• value — apply as the override for every segment.

Call setStyleOverride({ color: null }) to clear just the colour override.

Contrast OverridableStyle when used as LineSource.styleOverride: a stored override where a field is simply present (applied) or absent — the whole override is cleared by assigning null, not per field.