Graphics problems in XCOM: Enemy Unknown using an ATI Radeon HD 4850 with open source radeon driver in debian jessie

[Bob]

So I'm running Debian jessie, using an ATI Radeon HD 4850 with the open source radeon drivers, and I'm getting very messed up graphics in XCOM: Enemy Unknown/Within.

I've tried changing the graphics settings in-game and seems to make no difference, not sure what else I should be looking at. Also not sure if this is just with this game, I've not tried any other graphically intensive games on this PC, but can install something else if needed.

If it makes a difference the actual OS installed is Crunchbang Waldorf (which is just Debian Wheezy with a few minor things added/removed), and I then changed the sources used to the Jessie/testing ones. Here's some pictures of the glitches: http://imgur.com/a/PPlGN

Any advice anyone? Thanks