Adds camera.js with perspective, lookAt and matrix helpers. The orbit
view is now a lookAt pose feeding one view-projection matrix shared by
the shaders and project(), replacing the hand-rolled projection with
its clamped depth. A node test checks the new camera reproduces the old
projection to within 0.05 px, so the landscape looks the same; this is
the base the city's walk and fly cameras need (near plane, correct w).
