added Fill method

write_string.go

@@ -10,3 +10,12 @@ func (b *Buffer) WriteString(str string, x, y int) {
 		b.Set(x+dx, y, r)
 	}
 }
+
+// Fill fills the whole buffer with c
+func (b *Buffer) Fill(c rune) {
+	for _, col := range b.data {
+		for ri := range col {
+			col[ri] = c
+		}
+	}
+}