radeonsidriver (the part of Mesa that implements OpenGL for recent AMD GPUs), you'll probably run into problems that can be fixed by starting Steam as
LD_PRELOAD=/usr/lib/i386-linux-gnu/libstdc++.so.6 steam(You'll have to have installed certain 32-bit packages, which you should have automatically been prompted to do while installing Steam.)
The background of this issue is that both Steam and
radeonsicontain C++ code and dynamically link against the C++ runtime libraries. However, Steam comes with its own copy of these libraries, which it prefers to link against. Meanwhile, Ubuntu 15.10 is built using the most recent
g++, which contains a new C++ ABI. This ABI is not supported by the C++ libraries shipped with Steam, so that by default, the
radeonsidriver fails to load due to linking errors.
The workaround noted above fixes the problem by forcing the use of the newer C++ library. (Note that it does so only for 32-bit applications. If you're running 64-bit games from Steam, a similar workaround may be required.)