Sprite referencing and removal in sprite atlas

[nickidlugash]

@kkaefer and I briefly discussed the possibility of implementing sprite removals from the sprite atlas – similar to the implementation in the GL Native glyph atlas of refcounting glyphs and removing when not being referenced by visible tiles.

This seems like it would be helpful for styles using large icon sets where the icons are localized, like highway shields or subway logos (i.e. where only small subsets of the icon sets need to be rendered for any given view).

/cc @mapbox/gl-core

[nickidlugash]

Main ticket for atlas improvements: #141