Add symbol-placement: fill-area

[samanpwbb]

A new symbol-placement value called fill-area would be useful in more stylized maps, for hand-placed looking hillshades and trees. For example of the kind of effect I'm after, see https://www.mapbox.com/bites/00099/ or https://www.mapbox.com/blog/super-mario-with-studio/. Here's how it would work:

[ajashton]

What happens when you zoom?

Seems like there's some overlap with https://github.com/mapbox/mapbox-gl-style-spec/issues/427

[samanpwbb]

More icons would appear as you zoom to fill space. The advantage of this over #427 is that there'd still be symbol collision detection, and we'd never render partial symbols.

[lucaswoj]

This issue was moved to mapbox/mapbox-gl-js#4127