wayland/display: Fix mmap use

[samueldr]

From version 7 onwards, the fd must be mapped with MAP_PRIVATE by the recipient, as MAP_SHARED may fail.

• https://wayland.freedesktop.org/docs/html/apa.html#protocol-spec-wl_keyboard

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Hi! :wave:

I have tracked down an issue wherein on aarch64-linux, rofi wouldn't take any input from niri down to this mmap use.

Without this change, rofi stays in focus, and only using clicks or killing it does anything useful.

Any attempt to type would result in:

May 04 20:59:58 lumpy rofi[8874]: nk_bindings_seat_handle_key: assertion 'self->keymap != NULL' failed

This does not affect a system with the same configuration on x86_64-linux, so when reproducing YMMV.

[lbonn]

Thanks for tracking this down!