Label zoom out improvements

[bcamper]

Improvements to label behavior on zoom out:

• Run collision more frequently (don't block on user input).
• Allow proxy labels to keep displaying, but only if they were previously visible (e.g. collision can remove visible proxy labels, but not add new ones that weren't already visible).
• Clamp curved label zoom interpolation factor (so curved labels don't go wild when zoomed beyond 1x up/down).
• Don't fade proxied labels out when zoomed beyond 1x (was only there to hide unsatisfactory proxy collision behavior addressed by above changes).