update race list when switching from tutorial to racing and back
Merge branch 'master' of https://github.com/nopoe/TuxRacer-SDL2
center-justify race and back buttons on race selection screen
submit score now called in game over screen
Merge branch 'master' of https://github.com/nopoe/TuxRacer-SDL2

Conflicts:
	src/platform.c

I think I fixed the conflict