Add Debug::check_assertion() overload that takes const char* parameter.

This avoids useless conversions to std::string when the assertion is
true.
Coding style.
Remove tabs
Main loop: ignore slow check_input() calls to compute the lag.

Indeed, slow check_input() are very sporadic, they typically happen when
loading a map. Unlike slow update() and slow draw(), they do not mean
that next calls will be slow.
Merge branch 'master' into texture