Make SurfaceLink an io.Closer

internal/system/system.go

@@ -1,5 +1,6 @@
 package system
 
+import "io"
 import "image"
 import "git.tebibyte.media/tomo/tomo/canvas"
 
@@ -23,6 +24,7 @@ type BackendLink interface {
 // SurfaceLink wraps a Surface created by the backend implementation, allowing
 // the System a higher level of control over it.
 type SurfaceLink interface {
+	io.Closer
 	GetSurface () any
 	SetSize    (image.Rectangle)
 }