Get snapping to character grid working again, avoid crashes when attempting
to resize below 1x1 character cell, only redraw when size actually changes.
Also, rename window_get_child_rectangle() to window_get_child_allocation().
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.
The dnd client now sends a mime type of
application/x-wayland-dnd-flower instead of text/plain. The dnd
message is just the position and random seed that lets the drop source
add the flower to its grid.
