Pack/unpack the tile planes arithmetically.

Pack/unpack the tile planes arithmetically.

Instead of a bunch of if statements, add a macro to extract an
individual tile value from the game/goal bitmaps using bitwise
arithmetic, and restructure the drawing paths to use it.

A similar change is made to populate the bitmaps during game board
initialization.  This simplifies the code a bit and also seems to
reduce the compiled size somewhat.