Wire demo, patch 1

A bugfix for v0-alpha.1, fixing crashes in specific situations.

This is not a completely functional demo, but it demonstrates the basic wire building mechanics and the ability to control the camera.

Controls

• Move the camera - Arrow keys, or hold middle mouse button and drag.
• Zoom in and out - Page Up / Page Down, or scroll mouse wheel up/down.
• Place a pin, or toggle between pin and crossover - Left click on a tile
• Place a wire - Left click and drag
• Remove everything on a tile - Right click on the tile