Created attachment 137961 [details]
terminal log of "MESA_GLSL=dump R600_DEBUG=nosb,vs,ps wine steam.exe"

I'm attaching full log created with
"MESA_GLSL=dump R600_DEBUG=nosb,vs,ps wine steam.exe &> glsl_error.log"

The most relevant line in the output seems to be this one:

"error: declarations for shader output `gl_BackSecondaryColor` are in
gl_PerVertex and gl_PerVertex"

It looks like the bug is not related to the R600 driver, but to the
glsl-compiler.