view: unconditionally center views in output layout

Whatever type or dimensions of a view, position it at center of the
output layout. Then, if it is a primary view or if it extends the output
layout, send request to maximize it.
The client may or may not change the view dimensions after that.
If not, e.g. weston-flower, the view will be kept displayed at center
instead of to left.
If so, the view will expand from center to fill the whole output layout.

In order to update the view position, XDG shell surface commit is now
handled to check for dimension change.

view.h

@@ -29,6 +29,7 @@ struct cg_view {
 
 	/* The view has a position in layout coordinates. */
 	int lx, ly;
+	int width, height;
 
 	enum cg_view_type type;
 	const struct cg_view_impl *impl;