Draw ball as a round-ish sprite.

This actually *decreases* resource usage, as apparently the
XST compiler is not particularly good at common subexpression
elimination and manually writing out the bit-level equivalent
of the position checks saves two 10-bit comparators.
Introduced GameController layer to hold high-level game logic.
Separate reset for game physics.

Preparation for supporting multiple lives.
Reset switch.

Reflashing via JTAG is still fast enough for testing though.
Ball loss detection.