It was impossible to export a small crop upscaled to 1200px wide to
demonstrate the effects of noise reduction, so I bumped the max scale
from 4 to 16 (to match the 1600% main preview max zoom).
This adds a little helper class to `guiutils.*` that unregisters
in-flight idle functions queued by `IdleRegister::add()`. It's best
to call `IdleRegister::destroy()` in the destructor of the class
owning the `IdleRegister` instance. Otherwise make sure, it is the
last member which will be deleted first.
`Resize` now makes use of this new facility in `setDimensions()`, which
also fixes#3673.
