Well, since nobody seemed to know, or cared to tell me . . .

I did some research myself, and figured it out!!!

The problem is
not that the Mesa libraries were corrupted, but rather that the
ati graphics driver that (K)Ubuntu supplies by default can't handle fonts in games made with Crystal Space, correctly. It also can't refresh the screen properly when you have multiple viewpoints, like in the 3D modeling program Blender. Most folks here wouldn't care too much about that, I guess, but it was while trying to solve the graphics corruption in Blender that I found out how to fix the font problem! The solution is to install the proprietary ATI driver, called
fglrx. Here's what you have to do (if you have an ATI card like I do, at least, don't know whether it works for others):
1). Download
this file. It is an automatic driver-installation program called Envy.
2). Open a terminal, and go to the directory you downloaded the file to (if you put it in your home folder, you'll start out in the right place automatically). Do this by typing:
cd path_to_envy's_deb_package replacing it with the proper folder names, of course.
3). Then type:
sudo dpkg -i envy*.deb4). Wait for it to finish, and then type:
sudo apt-get install -f5). Run Envy from the command-line, by typing:
sudo envy -t You'll have to enter your password before running the program. Once it's started up, press "3" to install the ATI fglrx driver, and allow it to update your xorg.conf and restart your computer.
6). Enjoy Planeshift!

Hope that helps anyone who's had the same problems as I have,
-Mark