The general idea is to generate fewer, separate boxes used for placement
and only generate glyphs for rendering after features have been placed.

This commit breaks lots of things.

like highway shields

Conflicts:
	js/data/create_bucket.js
	js/source/vector_tile_source.js
	js/source/worker.js
	js/source/worker_tile.js
	js/symbol/collision.js

For overscaled tiles, line label interpolation now places labels in the
same places it placed them in the parent tile, so that when you zoom the
labels don't shift.

…ry angle func

Keep upright

https://github.com/mapbox/bboxify-labels/
at e432c592f25c2a89bc87861adabd02a2a3575729

use a window of angles instead of a single angle. fixes #551

create separate AtlasImage type for images, instead of just added extra
properties to an image rect.

fixes #1033

Start another redoPlacement request only when the previous one has
finished.

Conflicts:
	js/data/symbol_bucket.js
	js/render/painter.js
	js/source/vector_tile_source.js
	js/source/worker.js
	js/source/worker_tile.js
	js/style/style.js
	js/symbol/glyph_source.js
	shaders/icon.vertex.glsl
	shaders/sdf.vertex.glsl

See the comment in the commit for more details.

Labels should mostly not overlap in perspective view.

The problem is that the angle from the camera to the surface is
different at the lower edge compared to the upper edge. This means that
labels near the upper edge cover a larger area than at the bottom edge,
even after they have been scaled for size. The only way to really fix
this is to redo placement when panning in the y direction.

The current implementation seems like it might be ok, so let's try this
for a while and see how it works.

Otherwise minScale is sometimes NaN and then GlyphVertexBuffer.add can't
be optimized ("optimized too many times" warning).