game_defs: Add scaled size/speed/location values

These are for the embedded port, which has a resolution of 40% of the
original.
screen: Add embedded screen dimensions and padding
Fix program memory copy offset bug in image and screen

Byte offsets and dword offsets were being mixed together.
shot: Deactivate based on defined values

Don't use hardcoded values.
screen: Enable sprite transparency
Initialize sprite sizes for all game entity types

Add sprite dimensions to game_defs.h
Add sprite dimension fields to game entity type properties
In game.cpp, move sprite allocation down to after sprite dimension are
set.
In screen.cpp, initialize the sprite dimensions.
game entity: add #define for alien vertical movement
screen: Implement vertical syncing for embedded code

Add a begin_update() to wait for the start of vertical blanking.

Use update() to wait for the end of vertical blanking.