compiler/glsl: handle case where we have multiple users for types

compiler/glsl: handle case where we have multiple users for types

Both Vulkan and OpenGL might be using glsl_types simultaneously or we
can also have multiple concurrent Vulkan instances using glsl_types.
Patch adds a one time init to track number of users and will release
types only when last user calls _glsl_type_singleton_decref().

This change fixes glsl_type memory leaks we have with anv driver.

v2: reuse hash_mutex, cleanup, apply fix also to radv driver and
    rename helper functions (Jason)

v3: move init, destroy to happen on GL context init and destroy

Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Timothy Arceri <tarceri@itsqueeze.com>
Reviewed-by: Jason Ekstrand <jason@jlekstrand.net>