plugins/screencast: fix damage region position

Damage reported from the output is output-local and therefore doesn't need to be translated.

I tested this with Chromium, where the damage region on my 3-monitor setup was always -4k something on the right screen, -2k on the middle one and correct on the left screen. With this change I can confirm that the position is now correctly reported for the right screen.

Fix suggested by @davidedmundson and @zamundaaa