Ensure the shared symbol table levels are read-only to make multi-threading safe. Also removed inadvertent extra copies of the symbol table shared across all stages.

git-svn-id: https://cvs.khronos.org/svn/repos/ogl/trunk/ecosystem/public/sdk/tools/glslang@22939 e7fa87d3-cd2b-0410-9028-fcbf551c1848

Todo.txt

@@ -1,10 +1,5 @@
 Current functionality level: ESSL 3.0
 
-Performance
-
-Testing
-  - thread safety
-
 Link Validation
     - ensure no static references thrown away
     Cross-stage linking
@@ -21,6 +16,8 @@ Link Validation
     Intra-stage linking
       - exactly one main
       - type consistency check of uniforms, globals, ins, and outs, both variables and blocks
+      - value checking of global const initializers
+      - value checking of uniform initializers
       - location/component/binding/index/offset match check
       - location/component aliasing (except desktop vertex shader inputs)
       - location layout range/overlap semantics