add CLI option to force release capture

Arimilrequested to merge
arimil/spectacle:master into master

Adds a CLI option to perform the equivalent of the Accept on click-and-release setting in the options. The CLI option will take precedence over the setting.

Example usage:

spectacle -bnrko /dev/stdout | catimg -

This allows spectacle to fulfill the use case of https://github.com/emersion/slurp but for kwin instead of wlroots.

Edited by Arimil

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