Hex Map 25: Exporation Bug Fix

One thing I failed to mention in Hex Map 25 is that the map debug visualization interferes with the exploration system. We're using the blue channel, which is also used to indicate whether a cell's visibility is in transition. This becomes an problem when moving units around a freshly-generated map. Cells with maximum debug values of 1 refuse to become visible.

I've decided to eliminate this issue, by changing SetMapData to use a maximum of 254 instead of 255. This keeps exploration functional. Unit movement will wipe out the debug data, which is fine.

The change is an extra step after creating HexCellShaderData.SetMapData, in section 1.1 Visualizing Data.