wayland backend needs to take into account monitor scale

On Wayland (with either screen capture protocol), I get quarter-cropped images on a 2x UI scale monitor because xfce4-screenshooter calculates the image buffer size based on logical dimensions, not physical.

Unfortunately the wlr-screencopy protocol doesn't actually say what coordinate space to use. ext-image-copy-capture does specify buffer coordinates. Grim appears to expect physical/buffer coords in both cases (and IIRC wl-screenrec crashes for me when using wlr-screencopy, but uses buffer coords for ext-image-copy-capture).