compute unique clip ids per tile, taking covering children into account

fixes #206
fixes #220
refs #229
add missing file
don't duplicate function
don't trip up certain stdlib implementations
add != operator for certain stdlib implementations
add a few fixes for gcc

fixes #437
remove unused threadpool class
never place infinitely deep symbols
also use boost codecvt on linux
use correct implementation
use platform-specific NSString for case changes on OS X/iOS

refs #309
add uppercase/lowercase symbols to headless test
use submodule version for ios build
increase build concurrency
Support for Mapbox fontstack API
Don't try to upload build artifacts for PRs
Set public-read ACL
Merge pull request #437 from mapbox/fix-gcc

Make linux fails
Merge pull request #440 from mapbox/infinite-placement

Never place infinitely deep symbols
Merge pull request #439 from mapbox/clipping

Compute unique clip IDSs per tile