Reorder TileLayer. (#5638)

Reorder TileLayer. (#5638)

Despite the split of TileSpec into TileDef and TileLayer, the
TileLayer struct is still 66 bytes large, and doesn't fit in
a single cacheline.

I'm moving the color member to cacheline 2, in the hope that it
is less used and the compiler loads all the hot members in a single
cacheline instead. Only color sits now in cacheline 2, all the
other members are in cacheline 1.

Note: is_color is probably rarely set, most nodes will likely
not use hardware coloring, but this may change in the future.
Ideally, this class is shrunk to 64 bytes.