Add blank vars to ensure layouts satisfy tomo.Layout

layouts/grid.go

@@ -4,6 +4,8 @@ import "math"
 import "image"
 import "git.tebibyte.media/tomo/tomo"
 
+var _ tomo.Layout = Grid { }
+
 // Grid is a layout that arranges boxes in a grid formation with distinct rows
 // and columns. It is great for creating forms.
 type Grid struct {