[Freeciv-Dev] (PR#3937) replace "2" with OVERVIEW_TILE_WIDTH

["Jason Short" <jdorje@xxxxxxxxxxxxxxxxxxxxx>]

The attached patch introduces two new variables, OVERVIEW_TILE_WIDTH and
OVERVIEW_TILE_HEIGHT.  These are used in place of "2" when doing all
overview calculations.  This is a minor step toward unifying some of the
overview code, as well as a potential feature in its own right.

They are variables (rather than constants) because gui-sdl already does
some basic recalculation of "optimal" overview tile size.  Eventually
this can be extended to something more general - perhaps even making
these values user options.

The patch is tested under some, but not all, clients.

jason