add a static assertion to ensure that the platform compiles right-shifts to sar.

src/main.c

@@ -26,6 +26,10 @@ static void quit(void)
 	glfwTerminate();
 }
 
+#if __has_extension(c_static_assert)
+__extension__ _Static_assert(-3 >> 5 == -1,
+	"The platform does not compile right-shifting signed integers to an arithmetic shift!");
+#endif
 
 /* Entry-point of the application. */
 int main(int argc, char **argv)