`view_zoomdrag_init()` would perform a number of sanity checks that
`view_zoom_poll()` already executed. The design of operators forsees that
(non-expensive) context sanity checks are done by the `poll()` callbacks, and
the execution callbacks can then just assume that happened. No reason to be
overly pedantic, we don't do that elsewhere either.
Note that this code is from the earliest days of an operator design. So it's
not surprising that it wasn't using the design "properly".
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