mesa: use accessors for struct gl_debug_state

mesa: use accessors for struct gl_debug_state

When GL_DEBUG_OUTPUT_SYNCHRONOUS is GL_TRUE, drivers are allowed to log debug
messages from other threads.  That requires gl_debug_state to be protected by
a mutex, even when it is a context state.  While we do not spawn threads in
Mesa yet, this commit makes it easier to do when we want to.

Since the definition of struct gl_debug_state is no longer needed by the rest
of the driver, move it to main/errors.c.  This should make it even harder to
use the struct incorrectly.

v2: add comments for the accessors

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