glsl: protect glsl_type with a mutex

glsl: protect glsl_type with a mutex

glsl_type has several static hash tables and a static ralloc context.  They
need to be protected by a mutex as they are not thread-safe.

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=69200
Signed-off-by: Chia-I Wu <olv@lunarg.com>
Reviewed-by: Brian Paul <brianp@vmware.com>
Reviewed-by: Ian Romanick <ian.d.romanick@intel.com>