add padding to annotation tiles

[ansis]

In order to properly sort markers across tiles (#988) we need annotation tiles to also include features that are outside of the tile's boundaries, but within a certain distance of the tile edge.

This will let us clip markers to tile boundaries at draw time to avoid incorrect overlap across tiles. Since markers near edges are included in both tiles they will be drawn seamlessly. Half the marker will be drawn by one tile and half by the other. This is how the -js implementation works.

This should be done after #1666 and #1655 are merged.

This will fix

After implementing, change these two lines:

mapbox-gl-native/src/mbgl/renderer/symbol_bucket.cpp

Line 326 in 3dfa317

// TODO remove the `&& false` when is #1673 implemented

mapbox-gl-native/src/mbgl/renderer/painter_symbol.cpp

Line 122 in 3dfa317

// TODO remove the `|| true` when #1673 is implemented

[incanus]

This is stale, right @ansis?

[ansis]

I think this still needs to be done. But it is unblocked now