[TD] Low Tiberium Growth on Megamaps

[ChthonVII]

The tiberium growth logic for TD in MapClass:Logic() selects a fixed, very small number of cells to attempt to grow or spread each cycle. (1 to grow and 1 to spread in single player; 2 x the lobby setting each in multiplayer.) This feels very anemic on megamaps where the same fixed amount of growth is spread out over 4x the area. And this is exacerbated by the fact that the selection process is biased and favors cells near the start or end of the array, potentially starving the middle of the map of any growth at all.

The fix is to scale the growth amount based on the ratio of the playable area of the megamap versus the 64x64 size the fixed growth amount was intended for.

Here's a CFEPatch Redux commit demonstrating the fix.