Add blank vars to ensure layouts satisfy tomo.Layout

layouts/contract.go

@@ -3,6 +3,8 @@ package layouts
 import "image"
 import "git.tebibyte.media/tomo/tomo"
 
+var _ tomo.Layout = ContractVertical
+
 // Contract is a layout that arranges boxes in a simple row or column according
 // to their minimum sizes.
 type Contract bool