Main: remove "using namespace std"
Keep stack of ints rather than function pointers

Translate the state codes to function calls in the main loop.  Function
pointers are icky.
Main: Put all globals and functions into a game class

Makes memory management easier on embedded systems.

Renamed all variables to have "m_" prefix instead of "g_" prefix.
Go directly into game, and then directly exit

Don't bother with menus for now.
Stub out Font rendering
Create Screen class as a wrapper for SDL video/font
Use int instead of enum to store block type