Whoops, looks like I ran into a bug or something in Metal specific to the video card I'm using: http://stackoverflow.com/questions/31925440/metal-texture-mapping-on-mac-os-with-swift …
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@MichaelTackes turned out to be a bug in the shader compiler.https://twitter.com/warrenm/status/688454553327423488 …
@robinkunde @MichaelTackes A better (?) workaround is just to make position a 4-vector rather than a 3-vector everywhere.