Please add dependency on libtxc-dxtn-s2tc0, required by various games

[Impact]
Various commercial games (including several from the Humble Bundle) require s3tc texture support. However this is not available with Intel graphics due to patent issues. Users with Intel graphics must manually install libtxc-dxtn-s2tc0 to make the games work.

The problem is compounded if the user is on amd64 and the game in question is 32-bit, because they then need libtxc-dxtn-s2tc0:i386, but there is no mechanism via the GUI to install this.

[Fix]
Add Recommends on libtxc-dxtn-s2tc0 to libgl1-mesa-glx. This will pull in the library directly on amd64 if the game is 64-bit, or will pull it in indirectly when the 32-bit mesa is installed if the game is 32-bit.

[Test Case]
1. On an Intel graphics system install 64-bit (amd64)
2. Install one of the affected games. (E.g. Bastion, Psychonauts, Amnesia, etc.)
3. Launch game

Expected Behavior: Game launches normally
Actual Behavior: Terminates with messages about s3tc texture support or missing GL support, or runs but graphics are messed up (missing textures/black surfaces.)

[Regression Potential]
None really, this merely causes one additional package to be installed that is already available in the archive.

Perhaps if someone has the patent-encumbered libtxc_dxtn installed this could conflict, however we do not provide that package in the archive.

[References]
All the following are examples of users on Intel gfx / 64-bit that ran into problems running Humble Bundle games, and reportedly solved it by installing the libtxc-dxtn-s2tc0 package.

"So I ran some code that updated my Ubuntu Software Center to include Bastion. But now when i try to download and install i get this error message: Depends: libtxc-dxtn-s2tc0 but it is not going to be installed Anyone know a fix?"
--- http://ubuntuforums.org/showthread.php?t=1992367&page=15

"When I try to load Psychonauts it crashes and this is the result I get when running from teminal. Any help or suggestions are greatly appreciated.
ERROR: Missing required OpenGL extensions:
- GL_EXT_texture_compression_s3tc"
--- http://askubuntu.com/questions/145522/psychonauts-humble-indie-bundle-v-error

"After installing Bastion and starting it up, only it shows first a few logos, and then only a black screen and the mouse pointer."
--- http://askubuntu.com/questions/145962/bastion-on-ubuntu-12-04-64-bit-gives-only-a-black-screen

"Amnesia just closes after I select New Game"
--- http://askubuntu.com/questions/146587/amnesia-just-closes-after-i-select-new-game