games/marsnomercy: Fix build with Clang 6

src/mars.cpp:423:29: error: non-constant-expression cannot be narrowed from type 'int' to 'Sint16' (aka 'short') in initializer list [-Wc++11-narrowing]

SDL_Rect mm_limit_1024 = { (mm.GetW() - (mm_border_1024->GetW() - 2)) / 2, 0,
                           ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~

http://beefy12.nyi.freebsd.org/data/head-amd64-default/p475044_s336572/logs/marsnomercy-0.2.1_12.log

• Add missing libpng dependency