protocol: Add wl_surface.offset

This is meant to address the issue where the call to 'wl_surface.attach'
is done by e.g. Vulkan, meaning applications cannot affect the values of
the offset passed as the x and y arguments.

The lack of ability to pass these is problematic when using the Vulkan
for e.g. drawing DND surfaces, as the buffer offset is used to implement
the drag icon hotspots.

Closes: wayland/wayland#148
Signed-off-by: Jonas Ådahl <jadahl@gmail.com>
Reviewed-by: Simon Ser <contact@emersion.fr>
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