Ensure worker tasks are complete before destroying TileData

[jfirebaugh]

This is the sure-fire fix for #1309. Should have gone this route earlier.

[incanus]

README is going to need updated for C++14 req.

[springmeyer]

You mentioned at #1403 (comment) that only clang-3.4 is required. I see you bumped to clang-3.6 - is that needed specifically or did you just bump to the latest clang available for good practice? The reason I ask is that I've been noticing crashes which look like some kind of ABI incompatiblity when c++ binaries built with clang-3.5 are linked and run against other c++ binaries built with clang-3.6 mapnik/mapnik#2785. I've not had a chance to investigate much but until I do I've been sticking with clang-3.5.

[jfirebaugh]

@incanus README update is included.

@springmeyer Previously I encountered a conflict between gcc 4.9 and clang 3.4/3.5. But the Travis infrastructure has changed in the meantime and this may no longer be an issue. I'll try reverting back to 3.4.

[jfirebaugh]

clang-3.4 does not seem to support -std=c++14. I'll try 3.5.

[incanus]

@jfirebaugh Would this be accurate to change as well in the README?

An OpenGL renderer for Mapbox Vector Tiles,
implemented in C++14, currently targeting iOS, OS X, Android, and Ubuntu Linux.

[jfirebaugh]

Yes, I'll do that.