const PITCH_LIMIT: number
How close to vertical the view may pitch, radians.
ELEVATION_LIMIT itself, not a restatement of it: the argument and the
margin are the same — lookAlong levels the horizon against the local up,
so a view aimed exactly along it has no horizon to level and rolls a half
turn in one pointer-pixel — and two constants that must agree drift the
first time one is retuned. Two degrees is invisible and removes the
singularity instead of special-casing it.