Update code style

Limit source code to 80 chars, and declare functions return type and
modifiers on a separate line.

This allows to avoid very long lines, and all function names are
aligned.

(We do this on VLC, and I like it.)

app/src/tiny_xpm.h

@@ -3,6 +3,7 @@
 
 #include <SDL2/SDL.h>
 
-SDL_Surface *read_xpm(char *xpm[]);
+SDL_Surface *
+read_xpm(char *xpm[]);
 
 #endif