added support for zero-sized buffers

buffer.go

@@ -102,8 +102,8 @@ func (b *Buffer[T]) Sub(x, y, w, h int) *Buffer[T] {
 	// sanitize inputs
 	x = limit(x, 0, b.width-1)
 	y = limit(y, 0, b.height-1)
-	w = limit(w, 1, b.width-x)
+	w = limit(w, 0, b.width-x)
-	h = limit(h, 1, b.height-y)
+	h = limit(h, 0, b.height-y)
 
 	// make slice references
 	data := make([][]T, h)