main: Report start and end of stack
Move game entity type definitions to separate file

This allows the type definitions to be included without having to include
game_entity.h.
images: Add image offset lookup function
game: specify 32-bit counters, decrease reporting to 100 loops
screen: Allocate and initialize sprites

Add a function to assign sprite indexes to game entities.  This data is
stored as a list of first sprite index for each entity type.
Rename palette data to have generic name

Previously it was taken from one of the alien images, so the pointer and
size had names that reflected the alien image file.  However, the palette
is to be used across all images.
screen: set up palette
screen: implement update_sprite()
game: move various #defines into new header file

Keeps game.cpp smaller and cleaner.
Also allows the definitions to be shared with other source files.