Don't mark items that are outside the collision grid range as placed.
Requires new ignore because GL JS issue #5654 allows insertion of symbols outside the CollisionIndex range, and those symbols can cascade in to affect items within the viewport.
- Background placement code now just generates static symbol buffers
- Don't render GeometryTiles until their symbols are loaded. This is necessary for the CrossTileSymbolIndex to successfully prevent flicker.
- SymbolInstances are transferred to SymbolBucket for use on foreground during collision detection
- Symbols are sorted on foreground by sorting their index buffer but leaving vertex buffers intact (only works within one segment)
- Vertical glyphs are generated at same time as horizontal glyphs. `reprojectLineLabels` chooses which one to use at render time and hides the other.
- Icons are now always represented with a single collision box, even if they're placed along a line (this means their rotation alignment may be wrong, but the approach of representing them with multiple collision boxes wasn't very accurate either).
- Generate vertices for new debug collision boxes and collision circles
- Only add symbols within tile boundaries (reduces work, avoids double-draw)
- Update symbol_projection.cpp to support line label projection calls from CollisionIndex.
Pad each icon with one pixel on the bottom and right sides.
On top of this, extra padding is added to round the widths and heights
to multiples of four.
This then fixes the positions of the bottom and right vertices to
correctly center the icon's box around the center of the icon, not the
center of the padded icon.
js: 32ca4007699d1d21ab25bb38588b151510194030
fixes https://github.com/mapbox/mapbox-gl-js/issues/947
