layout: ensure Flex{Alignment: Middle} respects minimum constraint

Before this change, the middle alignment would align according to
the widest child. This change aligns according to the widest child
or minimum constraint, whichever is largest.

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

layout/flex.go

@@ -142,7 +142,7 @@ func (f Flex) Layout(gtx Context, children ...FlexChild) Dimensions {
 		children[i].call = c
 		children[i].dims = dims
 	}
-	var maxCross int
+	maxCross := crossMin
 	var maxBaseline int
 	for _, child := range children {
 		if c := f.Axis.Convert(child.dims.Size).Y; c > maxCross {