cleanup, fix crashes for some board-edge interactions

src/marble_engine/board.rs

@@ -108,15 +108,14 @@ impl Board {
 		}
 	}
 
-	pub fn get_mut_unchecked(&mut self, p: Pos) -> &mut Tile {
+	pub fn get_blank_mut(&mut self, p: Pos) -> Option<&mut Tile> {
 		if self.in_bounds(p) {
-			&mut self.rows[p.y as usize][p.x as usize]
-		} else {
-			panic!(
-				"position {p:?} out of bounds, size is {}x{}",
-				self.width, self.height
-			);
+			let tile = &mut self.rows[p.y as usize][p.x as usize];
+			if tile == &Tile::Blank {
+				return Some(tile);
+			}
 		}
+		None
 	}
 
 	pub fn set(&mut self, p: Pos, tile: Tile) {