Consider amphibious defenders

    When selecting an ideal defending unit against some attacker,
also consider amphibious units as possible defenders.

See patch #4644

git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24779 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Made AI to value coinage less than buildings that will forever produce same, or near
same, amount of money.

See bug #21796


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24780 a0f10bec-cc02-0410-94fc-a9cfff90b4cd
Fixed and enabled sdl2-client's get_smaller_surface_rect()

See patch #4634


git-svn-id: svn://svn.gna.org/svn/freeciv/trunk@24782 a0f10bec-cc02-0410-94fc-a9cfff90b4cd