create tile bounds buffers lazily

[ansis]

A potential optimization: we should be able to create tile bound buffers only for tiles that are in the 'loaded' state, on-demand for visible tiles. Creating the tile bounds buffer can be a non-trivial amount of work. These buffers are created in the tile constructor, but tiles may never reach the 'loaded' state, resulting in these buffers being created and destroyed wastefully.

mapbox-gl-js/src/source/tile.js

Line 173 in 5af4076

this._makeTileBoundsBuffers(painter.context, projection);