glsl: protect anonymous struct id with a mutex
glsl: protect anonymous struct id with a mutex

There may be two contexts compiling shaders at the same time, and we want the
anonymous struct id to be globally unique.

Signed-off-by: Chia-I Wu <fdb43c5ef76ecda0c1661d6d199b5bfac1db538a@lunarg.com>
Reviewed-by: Brian Paul <3cb4e1df5ec4da2c7c4af7c52cec8cf340a55a10@vmware.com>
Reviewed-by: Ian Romanick <2b237cafb16dc45038e85df6c85e74e6d899eba9@intel.com>

metora/MesaGLSLCompiler,metora/MesaGLSLCompiler,metora/MesaGLSLCompiler