Add grid-layer-surface

[vyivel]

The size is a hint, in the sense that the client is free to ignore it if
it doesn't resize, pick a smaller size (to satisfy aspect ratio or
resize in steps of NxM pixels). If the client picks a smaller size and
is anchored to two opposite anchors (e.g. 'top' and 'bottom'), the
surface will be centered on this axis.

This client checks if the compositor handles this case correctly.