Airspace: GetAirspace() returns reference
Util/{Const,Writable}Buffer: add template specialisation for <void>

Allows adding assertions to the non-void implementation.
OpenGL/Buffer: remove unused id constructor
OpenGL/FallbackBuffer: wrapper for GLBuffer with fallback

Allows using the same code for OpenGL implementations with and without
VBO.
TopographyFileRenderer: migrate to GLFallbackArrayBuffer