Check whether rescale is requested before checking is_scaled_image

[squidpickles]

What does this PR do?

This change checks whether do_rescale is enabled in image processors before checking is_scaled_image().

is_scaled_image() needs to scan every element to get the min and max values, to determine whether they are between 0 and 1. If do_rescale is False, this is an unnecessary check.

This change swaps the order of the check, so it first checks do_rescale and then is_scaled_image(). Just a minor performance improvement on some image processing.

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• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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@amyeroberts, @qubvel