Prevent Wayland crash in working-area X11 code path

[slowriot]

This PR fixes a crash when gpicview is run on non-X11 backends, notably native Wayland sessions. This is currently an issue on Gnome desktop in Debian Testing.

Cause

gpicview uses src/working-area.c to query the desktop work area when choosing its initial window size and zoom mode. That code assumes the active GDK backend is X11 and unconditionally calls gdk_x11_display_get_xdisplay() / gdk_x11_screen_get_xscreen(). On Wayland, those assumptions are invalid, which can lead to a crash when opening an image.

Fix

The fix is deliberately small: it keeps the existing X11 code path unchanged, but only uses it when the current display/screen are actually X11. On non-X11 backends, it returns the existing fallback path instead, which uses the screen dimensions.

• This solution preserves current behavior on X11.
• This solution avoids backend-specific calls on Wayland and other non-X11 backends.
• It reuses the fallback behavior the file already had, rather than introducing new platform-specific logic.

[Copilot]

Pull request overview

Fixes a crash when running gpicview on non-X11 backends (notably native Wayland) by avoiding unconditional X11-specific GDK calls in the working-area detection code.

Changes:

• Wrap X11 header usage with #ifdef GDK_WINDOWING_X11.
• Gate the X11 workarea query path so it only runs when the active GDK display/screen are actually X11.
• Fall back to the existing “screen dimensions” path on non-X11 backends.

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