If you have been waiting for Portal 2 to play on your Mac or PC now is the time. Geoff Keighley’s interactive app, originally an exclusive on the iPad, is now available to download via Valve’s Steam service for $1.99/£1.49. Link below. The Final Hours of Portal 2 takes you deep within the top-secret offices of Valve for an unvarnished look at the creative process behind the new video game Portal 2. Journalist Geoff Keighley was granted unprecedented “fly on the wall” access to Valve to create this staggering 15,000 word multimedia experience. From the hush-hush Portal prequel that was shelved to the last minute scramble to complete the game’s…