HDMI input isn't standard on most computers, but maybe your Mac is an exception?
A device like this might work?
Amazon.com: AGPtek® HD Game Capture HD
There are dozens of variants, this seems to be about the lowest in price. Search for video game capture devices (xbox, playstation, etc), because the only thing better than playing video games is being able to share footage of you playing video games with the world, so there is apparently a demand.
Here's a blog of someone who uses such a device for this purpose:
How to capture great looking camera LCD screenshots