Fix build with clang
https://bugs.gentoo.org/739090

--- a/src/SFont_gl.cpp
+++ b/src/SFont_gl.cpp
@@ -223,7 +223,7 @@ void Mafferyew::SFont_gl::create_font_char(SDL_Surface* image,
                                      0x000000ff, 0x0000ff00, 0x00ff0000, 0xff000000);
 #endif
     
-    SDL_Rect src_rect = { begin_x, 1, font_char_w, font_height };
+    SDL_Rect src_rect = { (Sint16)begin_x, 1, (Uint16)font_char_w, (Uint16)font_height };
     SDL_BlitSurface(image, &src_rect, converted, 0);
     
     GLuint texture;