Tidy up the code in game.cpp

[Daft-Freak]

• Set the pen before clearing (catches me out every time I start a new project...)
• Remove resetting unused alpha/mask
• Re-format comments
• Fix misleading "since the start of your game" (firmware startup has always taken time, now that there isn't a reset before launching the game this is more noticeable)
• Point out that update is always 10ms and lores is the default mode

... and I only came here to fix the first one...