Fix rendering of tiles with extent != 4096

[lucaswoj]

fixes #1952

regression caused by #1886, as predicted

[lucaswoj]

I still see a number of instances of the ✨ constant 4096 scattered around the codebase but rendering seems to be correct in all cases I can identify.

👀 @ansis @mourner

[ansis]

It would be good to remove more of the remaining 4096's. It looks like some of them will cause clipping, drouble-drawing and pattern-sizing issues. the instances in:

• circle_bucket.js
• symbol_bucket.js
• draw_fill.js
• get_anchors.js

I think some of the others should be removable too

[mikemorris]

Would be great to get this ported to gl-native too, happy to lend a hand if you want to give it a shot @lucaswoj.

Refs mapbox/mapbox-gl-native#250

[lucaswoj]

@mikemorris this feature was implemented by @tmcw in #1234, I'm just fixing a regression caused by layer-by-layer.

[lucaswoj]

Good call @ansis. Fixing those exposed some bugs in test suite.