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
coalesce texture uploads at the beginning of a frame
allow crosstile labels and add `symbol-avoid-edges`
use pimpl for collision in tile_parser
Merge pull request #423 from mapbox/coalesce-texture-uploads

Upload textures at the beginning of a frame
Merge pull request #419 from mapbox/crosstile

allow crosstile labels and add `symbol-avoid-edges`
remove support for composite layers
remove trailing whitespace + use local image sprite
fix stylesheet indentation
remove composite enums from tests