The new map_toplevel() request no longer specifies a position and takes
the size from the attached buffer.  The attach request now takes a
position relative to the top-left corner of the old buffer to let
clients specify the relative position of the new buffer.

This is an example of how to animate window resizing.  The size is
animated by a simple physics model that allows the user to cancel the
animation halfway and still look good.

Still very much hand-wavey, but at least it's only in one place now.

Now all grabs use grab objects, so drop the grab enum.

Fixes crash on click in terminal.

We'll need a better strategy for generating timestamps without input events
or pageflips.  At least we'll need to ensure everybody is using the same
underlying time source.

or if that check is removed - crash (unsurprisingly).
This patch relieves this assumption, and allow some clients (terminal,
image, flower) to run in a non-DRM environment

Instead of display->base.base we want display->proxy.object, or

        buffer->buffer.resource.object.id = id;

which makes it clear what we end up addressing.

Backend can now rely on their destroy function getting called on shutdown,
so reset tty etc there instead of handling SIGTERM twice.

Use the runtime dir from XDG Base Directory Specification for creating the
socket in a directory only the user can read and write.