add cli tool for rendering a map to an image
show default values and allow setting the output filename
add xcode project for render cli util
allow setting the cache database
don't provide a default value for the stylesheet file
fix crashes when running in non-continuous mode
set rendering mode so that we can skip intermediate renders for static images
fix error in non-debug builds
don't assing nullptr to std::string
never create core profile contexts; their API is subtly different

(e.g. glGetString(GL_EXTENSIONS) doesn't work, you'd have to use glGetStringi(GL_EXTENSIONS, ...);)
automatically copy the certificate bundle
Merge pull request #718 from mapbox/cli-render

CLI render