radeon: mir_demo_client_scroll fails to render with multiple windows

If you run mir_demo_client_scroll with multiple windows (e.g. mir_demo_client_scroll -w 2) on radeon (only r600 tested, it seems the problem lies in the Gallium layer) one of the windows will fail to render succesfully.

You can use GL_DEBUG=1 MESA_DEBUG=1 to print GL errors, and you will see it relates to the texture shader program failing to link, due to nonsense such as texture2D(sampler2D, vec2) not existing. The same program will succesfully link in the other thread! Adding a sleep of one second inbetween the multiple window threads, is enough to make the shader compile.

It seems then we have run in to some sort of thread safety issue in the Gallium shader compiler.