devel/flatzebra: unbreak build with Clang 6 (C++14 by default)

In file included from GameEngine.cpp:23:
../../src/flatzebra/GameEngine.h:412:23: error: non-constant-expression cannot be narrowed from type 'int' to 'Sint16' (aka 'short') in initializer list [-Wc++11-narrowing]

SDL_Rect rect = { x, y, width, height };
                  ^

../../src/flatzebra/GameEngine.h:412:23: note: insert an explicit cast to silence this issue

SDL_Rect rect = { x, y, width, height };
                  ^
                  static_cast<Sint16>( )

../../src/flatzebra/GameEngine.h:412:26: error: non-constant-expression cannot be narrowed from type 'int' to 'Sint16' (aka 'short') in initializer list [-Wc++11-narrowing]

SDL_Rect rect = { x, y, width, height };
                     ^

../../src/flatzebra/GameEngine.h:412:26: note: insert an explicit cast to silence this issue

SDL_Rect rect = { x, y, width, height };
                     ^
                     static_cast<Sint16>( )

../../src/flatzebra/GameEngine.h:412:29: error: non-constant-expression cannot be narrowed from type 'int' to 'Uint16' (aka 'unsigned short') in initializer list [-Wc++11-narrowing]

SDL_Rect rect = { x, y, width, height };
                        ^~~~~

../../src/flatzebra/GameEngine.h:412:29: note: insert an explicit cast to silence this issue

SDL_Rect rect = { x, y, width, height };
                        ^~~~~
                        static_cast<Uint16>( )

../../src/flatzebra/GameEngine.h:412:36: error: non-constant-expression cannot be narrowed from type 'int' to 'Uint16' (aka 'unsigned short') in initializer list [-Wc++11-narrowing]

SDL_Rect rect = { x, y, width, height };
                               ^~~~~~

../../src/flatzebra/GameEngine.h:412:36: note: insert an explicit cast to silence this issue

SDL_Rect rect = { x, y, width, height };
                               ^~~~~~
                               static_cast<Uint16>( )

Reported by: pkg-fallout (blocks 3 ports)