Add blank vars to ensure layouts satisfy tomo.Layout

layouts/flow.go

@@ -3,6 +3,8 @@ package layouts
 import "image"
 import "git.tebibyte.media/tomo/tomo"
 
+var _ tomo.Layout = FlowVertical
+
 // Flow is a grid layout where the number of rows and columns changes depending
 // on the size of the container. It is designed to be used with an overflowing
 // container. If the container does not overflow in the correct direction, the