Ensure BlockMap::clip_point always yield a valid buffer location

crates/text/src/point.rs

@@ -35,6 +35,14 @@ impl Point {
     pub fn is_zero(&self) -> bool {
         self.row == 0 && self.column == 0
     }
+
+    pub fn saturating_sub(self, other: Self) -> Self {
+        if self < other {
+            Point::zero()
+        } else {
+            self - other
+        }
+    }
 }
 
 impl<'a> Add<&'a Self> for Point {