layout: add "exp" format layout for expanding both constraints

Signed-off-by: Elias Naur <mail@eliasnaur.com>

layout/format.go

@@ -47,7 +47,7 @@ type formatError string
 // is one of north, northeast, east, southeast, south, southwest, west,
 // northwest, center.
 //
-// hexpand/vexpand(widget) forces the horizontalor or vertical
+// hexp/vexp/exp(widget) forces the horizontal, vertical or both
 // constraints to their maximum before laying out widget.
 //
 // hcap/vcap(size, widget) caps the maximum horizontal or vertical
@@ -131,6 +131,13 @@ func formatLayout(gtx *Context, state *formatState, widgets []Widget) {
 		ctxLayout(gtx, cs, func() {
 			formatExpr(gtx, state, widgets)
 		})
+	case "exp":
+		cs := gtx.Constraints
+		cs.Width.Min = cs.Width.Max
+		cs.Height.Min = cs.Height.Max
+		ctxLayout(gtx, cs, func() {
+			formatExpr(gtx, state, widgets)
+		})
 	case "hcap":
 		w := parseValue(state)
 		expect(state, ",")