The code checks for the camera-stage mapper matrix by looking at its truth value.
However, this is ambiguous because the matrix has many elements. That means that
if it's missing, we get the expected result, but if it's present
we get an error.
I have avoided dict.get() and instead look it up with [] notation.
This will raise a KeyError if it's missing, which I
handle helpfully.
If the key is present but the value is not castable to an array, we'll
get an exception anyway.
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