grid: Drop unused tilemap methods
grid: Rename some things
grid: Remove label_tile() assumption that tiles have a column/row
grid: Move read_tiles() callback pointer out of ops structure

The ops.read_tile method is only called by read_tiles(), and grid
implementations don't actually have to call read_tiles().  Pass the
callback function pointer as an argument to read_tiles() to avoid
implying that implementing the function is otherwise required.