Tile cleanup

Tiles textures are maintained inside the TileGrid alongside buffers and Servo is no longer involved in the creation of Textures.
I get this, but perhaps I'm doing it incorrectly:

    src/support/layers/rust-layers/tiling.rs:140:60: 140:64 error: cannot borrow immutable local variable `tile` as mutable
    src/support/layers/rust-layers/tiling.rs:140                 Some(tile) => self.add_unused_buffer((&mut tile).buffer.take()),