feat: add -O to print focused output geometry immediately

[mbroemme]

Description

Take a screenshot with grim using slurp of the currently focused display with the Print key. Press Print once to grab a screenshot of the monitor your mouse is currently on (the focused output), without selecting a region or clicking.

Why

On multi-monitor setups, "full-screen screenshot" often captures all outputs (huge image) or the wrong monitor. With slurp -O, the screenshot is always of the display you’re actively using.

Example:

Bind Print to:

sh -c 'grim -g "$(slurp -O)" ~/Pictures/Screenshots/$(date +%F_%H-%M-%S).png'

A PNG file is created containing exactly the focused output (its x/y origin and width/height), with no interactive selection step.

Implementation

Add a new -O option that queries the output currently containing the pointer and prints its box using the existing format string machinery, then exits without entering region selection. Reject -O when combined with -p or -r to avoid ambiguous behavior. This enables grabbing the active display dimensions instantly (e.g. for scripts) without requiring a click/drag.

[emersion]

I don't think showing a layer surface and then immediately closing it is a good way to get the focused output.

I think this should be handled on the capture side instead. See e.g. https://gitlab.freedesktop.org/wlroots/wlr-protocols/-/issues/116

[mbroemme]

Which basically means to replace wlr-screencopy with ext-image-copy-capture in grim, correct?