mesa: clean up debug output namespace handling

mesa: clean up debug output namespace handling

Add functions to provide these operations on a struct gl_debug_namespace:

  init():    initialize the namespace
  copy():    copy all elements from one namespace to another
  clear():   clear all elements (to free the memories)
  set():     set the value of an element
  set_all(): set the value of all elements
  get():     get the value of an element

A debug namespace is like a sparse array.  The length of the array is huge,
2^sizeof(GLuint), but most of the elements assume the same value sepcified by
set_all().

Signed-off-by: Chia-I Wu <olv@lunarg.com>
Reviewed-by: Brian Paul <brianp@vmware.com>