Center surface inside container when it's too small

The goal here is to center fullscreen views when they are both too small
for the output and refuse to resize to the output's dimensions. It has
the side effect of also centering the view when it's too small for its
container.

Example clients that have this behaviour are emersion's hello-wayland
and weston.

It works by introducing surface_{x,y,width,height} properties to the
container struct. The x and y represent layout-local coordinates where
the surface will be rendered. The width and height are only used to
track the surface's previous dimensions so we can detect when the client
has resized it and recenter and apply damage accordingly.

The new surface properties are calculated when a transaction is applied,
as well as when a view resizes itself unexpectedly. The latter is done
in view_update_size. This function was previously restricted to views
which are floating, but can now be called for any views.

For views which refuse to resize *smaller* than a particular size, such
as gnome-calculator, the surface is still anchored to the top left as
per the current behaviour.

include/sway/tree/container.h

@@ -88,9 +88,16 @@ struct sway_container {
 	double saved_x, saved_y;
 	double saved_width, saved_height;
 
+	// These are in layout coordinates.
 	double content_x, content_y;
 	int content_width, content_height;
 
+	// In most cases this is the same as the content x and y, but if the view
+	// refuses to resize to the content dimensions then it can be smaller.
+	// These are in layout coordinates.
+	double surface_x, surface_y;
+	double surface_width, surface_height;
+
 	enum sway_fullscreen_mode fullscreen_mode;
 
 	enum sway_container_border border;