Fix #169: account for effect of centre-of-pixel coordinates when zoomed.

Rectangles (including clip) start and end mid-pixel, so adjust by 0.5 accordingly.

Still not 100% sure this is all correct, but it's definitely an improvement.

If changing it, be sure to try high zoom on at least Map, Light Up, Mines, and Inertia.

Also anti-alias by default except for rectangles (puzzles drawing API requirement).

Fixed graphics overflowing square on mines, and tweaked bevel on inertia.