mesa: allocate gl_debug_state on demand

mesa: allocate gl_debug_state on demand

We don't need to allocate all the state related to GL_ARB_debug_output
until some aspect of that extension is actually needed.

The sizeof(gl_debug_state) is huge (~285KB on 64-bit systems), not even
counting the 54(!) hash tables and lists that it contains.  This change
reduces the size of gl_context alone from 431KB bytes to 145KB bytes on
64-bit systems and from 277KB bytes to 78KB bytes on 32-bit systems.

Reviewed-by: Reviewed-by: Kenneth Graunke <kenneth@whitecape.org>