Support fractional minzoom on layers

[asheemmamoowala]

Addresses : #2929

GeoJSON and Vector Tile sources are using a floor'ed zoom value when loading new tiles. This blocks tile data loaded for layers with fractional minzoom as it doesn't pass the test in worker_tile.js

The solution requires two changes

• Pass the current zoom level to the worker tile
• Worker tile should parse tiles that are at the integer min zoom floor

benchmark	master b528524	2929-fractional-minzoom 6d1c21b
map-load	180 ms	113 ms
style-load	107 ms	88 ms
buffer	1,057 ms	926 ms
fps	60 fps	60 fps
frame-duration	6.1 ms, 0% > 16ms	6.3 ms, 1% > 16ms
query-point	0.94 ms	0.98 ms
query-box	53.86 ms	49.46 ms
geojson-setdata-small	4 ms	3 ms
geojson-setdata-large	145 ms	137 ms

Checklist

• briefly describe the changes in this PR
• write tests for all new functionality
• document any changes to public APIs
• post benchmark scores
• manually test the debug page

[anandthakker]

@asheemmamoowala I'd love to avoid another point of dependency of sources on the map object (see also #3350).

Would an alternative to using the current map's zoom level be to keep using tile.coord.z, but change the test to something like if (this.zoom < floor(layer.minzoom)) continue;?

If I'm understanding this right, we want:

layer minzoom	tile zoom	tile needed?
4	3 (and below)	no
4	4 (and above)	yes
4.5	4 (and above)	yes
4.5	3 (and below)	no

[anandthakker]

Could we simplify this test a tad by:

• width & height: 64
• center 0,0
• data: literal geojson point

[anandthakker]

LGTM! Approving, notwithstanding a couple of small notes that I should have caught last time.

[anandthakker]

Shoot, I should have thought of this before, but: could we move this test to render-tests/regressions/mapbox-gl-js#2929?

[anandthakker]

Would it make sense to have two more layers with minzoom 4.5 and 5, that we would expect not to appear? (could use circle-translate so each layer's dot would be in a distinct location)

[asheemmamoowala]

That's a good improvement to the test, Thanks !

[kkaefer]

This needs implementation in Native.