bump Cocoa for bundled style API
only set changed glDepthRange values
use vaos only when the value is 1, not when it is merely defined
remove compositing from stylesheet
fix image pattern rendering
fix binding to vaos
add missing include
don't replace shaders on duplicate setup() call

fixes #408
refactor vao object to use acutal OpenGL IDs rather than object pointers
base static vertexbuffer on buffer object
Merge pull request #415 from mapbox/fix-vao

Line drawing slowdowns on OS X
allow crosstile labels and add `symbol-avoid-edges`
use pimpl for collision in tile_parser