glsl: Fix memory leak with known glsl_type instances

glsl: Fix memory leak with known glsl_type instances

When looking up known glsl_type instances in the various hash tables, we
end up leaking the key instances used for the lookup, as the glsl_type
constructor allocates memory on the global mem_ctx. This patch changes
glsl_type to manage its own memory, which fixes the leak and also allows
getting rid of the global mem_ctx and its mutex.

v2: remove lambda usage (Tapani)
    (+keep ASSERT_BITFIELD_SIZE, modify dummy ctor to initialize mem_ctx)

Bugzilla: https://bugs.freedesktop.org/show_bug.cgi?id=104884
Cc: mesa-stable@lists.freedesktop.org
Signed-off-by: Simon Hausmann <simon.hausmann@qt.io>
Signed-off-by: Tapani Pälli <tapani.palli@intel.com>
Reviewed-by: Emil Velikov <emil.velikov@collabora.com>
Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>